On Wednesday, Syria said it would abide by the ceasefire but reserved the right to respond to attacks.
The US said Syria's pledge held "little if any credibility".
The main armed rebel group said it would not stop shooting if government troops continued to attack.
Mr Annan received written assurances from the Syrian foreign ministry that government troops would "cease all military fighting throughout Syrian territory as of 06:00 (03:00 GMT)... while reserving the right to respond proportionately to any attacks carried out by armed terrorist groups", his spokesman, Ahmad Fawzi said.
'Regime won't stop'
Meanwhile, the violence continued on Wednesday, especially in the city of Homs.
Activists said at least 30 people were killed in offensives around the country, including the north and north-west, a valley west of Damascus, and in the south, near Deraa.
A spokesman for the main rebel force, the Free Syrian Army (FSA), said the ceasefire was unlikely to take effect because neither side would stop shooting.
Footage purporting to show clashes in Homs on Wednesday has been posted online
"If the other side stopped, the Syrian people would march on the president's palace on the same day. This means the regime won't stop," Captain Ayham al-Kurdi said in a BBC interview on the Turkey-Syria border.
The BBC's UN correspondent, Barbara Plett, says Mr Annan has been in contact with Iran, as well as Russia, asking them to use their influence.
If he fails, some Council members have said they will push for tougher action - not military intervention, but perhaps sanctions, our correspondent says.
However, there is no guarantee Russia and China would agree; they have already vetoed two Security Council resolutions on Syria.
Mr Annan is due to brief the UN Security Council later on Thursday.
The UN estimates about 9,000 people have died since anti-government protests began in March 2011. In February, the Syrian government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.
A tsunami watch declared after two major earthquakes off the coast of Indonesia's Aceh province has now been cancelled, the Pacific Tsunami Warning Center (PWTC) says.
Two hours after the quakes - one with a magnitude of 8.6, the other measuring 8.3 - the centre says "the threat has diminished or is over for most areas".
The alerts caused panic as people fled buildings and made for high ground.
There have been no immediate reports of damage or casualties. India, Thailand and Sri Lanka have also lifted their own tsunami warnings A PTWC alert said that sea level readings indicated a tsunami was generated and that it "may already have been destructive along some coasts," without specifying where.
A Thai disaster official said a 10cm wave had been recorded on Koh Miang island, off Phang Nga.
Earlier, Indonesian President Susilo Bambang Yudhoyono said the authorities were remaining "vigilant", despite the lack of tsunami reports.
"Our warning system is working well, and I have ordered the national relief team to fly immediately to Aceh to ensure the situation is under control and to take any necessary action," he said.
The alerts caused panic in Aceh province, as people fled from buildings
A few hours later, the PTWC renewed its warning after a major aftershock measuring 8.2 struck 16km (10 miles) beneath the ocean floor and 615km from Banda Aceh.
An AFP correspondent in Banda Aceh said the second aftershock lasted four minutes.
The PTWC issues advisory alerts across the region, which state authorities can use to issue their own emergency procedures. Indonesia straddles the Pacific Ring of Fire, a zone of major seismic activity.
'Minute of chaos'
Sutopo, a spokesman for Indonesia's disaster mitigation agency, said electricity had been cut in Aceh and there were traffic jams to access higher ground.
"Sirens and Koran recitals from mosques are everywhere," he told Reuters.
Tremors were felt as far away as Singapore, Thailand, Sri Lanka, Malaysia, Bangladesh and India. The French island of Reunion was also on alert.
"There was a tremor felt by all of us working in the building," a man called Vincent in Calcutta, India, told the BBC.
"All just ran out of the building and people were asked not to use the elevator. There was a minute of chaos where all started ringing up to their family and asking about their well-being."
Tsunami warning sirens, set up in many vulnerable areas after the 2004 disaster, were heard in Phuket, Thailand, where correspondents said people were calmly following evacuation routes to safe zones.
Roger Musson, a seismologist from the British Geological Survey, said the quakes were unlike those seen off Indonesia in recent years, where ground had been pushed under the continental plate, "flipping up" the seabed.
"It seems to be a large earthquake within the Indian Plate and the plate has broken in a sort of lateral way," he said.
"It's a sort of tearing earthquake, and this is much less likely to cause a tsunami because it's not displacing large volumes of water."
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(CNN) -- The 19-year-old Tulsa, Oklahoma, man whose Facebook page lamented his father's death "at the hands of a f--king n----r" told investigators he shot three of the African-Americans injured or killed in a Friday shooting spree, according to police documents.
Fellow suspect Alvin Watts confessed to shooting the other two people, according to the documents, which include only summaries of the reported confessions and few details.
Based on his statement, police believe Watts pulled the trigger in two of the three fatal shootings, according to the documents.
A police spokesman declined to comment Tuesday when asked about the reported confessions.
Authorities accuse the pair of gunning down apparent strangers at four different locations in a largely African-American section of Tulsa early Friday.
Authorities identified the victims as Dannaer Fields, William Allen and Bobby Clark. The other two shooting victims survived.
Police arrested Watts, 19, and Jake England, 33, early Sunday after tips led investigators to England's burned pickup. The vehicle matched one reported at the crime scenes, according to the arrest reports.
A judge ordered England and Watts held Monday on $9.16 million bond each pending formal charges on allegations of murder, shooting with intent to kill and gun possession.
Police reports characterize both men as white, but authorities have declined to say whether they think race played a role in the shootings.
An update posted to England's Facebook page the day before the shootings noted it was the second anniversary of his father's death "at the hands of a f--king n----r." The entry also mentioned his girlfriend's recent suicide.
Prosecutors will review whether hate crime charges are appropriate against England and Watts, Tulsa County District Attorney Tim Harris said Monday.
"If the motivation is racial in this case, then that needs to be vetted in a court of law just like any others," Harris said. "It's the law of the state of Oklahoma, and if the facts and the evidence support that, then we're going to go forward with it."
Police Chief Chuck Jordan described England and Watts as apparently close friends who shared a home in Tulsa.
One survivor of Friday's shootings, Deon Tucker, told reporters Monday that he was standing on his porch when a white pickup pulled up and the driver asked him for directions.
"Next thing I know, they start shooting," he said.
"What are they shooting me for? I ain't got no enemies. I didn't know what was going on," recalled Tucker, who said he was shot once in the chest.
He added that he understands that his race might have led to his shooting. But Tucker said he simply "got caught in the wrong spot" and isn't "mad at anybody," describing his shooter as a "lunatic."
On Friday, a friend had warned England not to "do anything stupid" after he posted a message Friday, apparently after the shootings, that read "It just mite be the time to call it quits."
"I hate to say it like that but I'm done if something does happen tonite be ready for another funeral later," England wrote.
"It's hard not to go off between that and sheran I'm gone in the head," he wrote, referring to his girlfriend.
The Facebook page was taken down Sunday afternoon.
"If that boy done this, there's no excuse for it," KTUL quoted Watts as saying. "I just want to apologize to those families. My heart goes out to them."
A man who identified himself as England's uncle -- speaking Monday outside the suspects' home -- said his nephew had been troubled by the recent suicide of his girlfriend and his father's death in April 2010.
"His mind couldn't take it anymore, I guess," the man said. "I guess it just snapped his mind."
Prosecutors declined to file homicide charges in the death of England's father, ruling it a justifiable homicide, the Tulsa County District Attorney's Office said in a statement Monday.
England and Watts remained jailed Tuesday pending an April 16 arraignment. Meanwhile, prosecutors prepared formal charges against the pair.
While Harris declined to say Monday if hate crime charges would be included, City Councilor Jack Henderson said the online posts and the circumstances of the shootings lead him to believe that the shooters targeted their victims chiefly because of race.
"You have an individual -- a white male -- going into a predominantly black community (to) shoot at black people. And with the other evidence ... and some of the things that were said, that leads me to believe that this was totally a hate crime," Henderson said.
Tulsa was the scene of a 1921 race riot -- considered one of the worst in the nation -- that destroyed the famed Greenwood District, a wealthy black enclave known as the black Wall Street.
Harris said the community's response to the shootings said more about current race relations in Tulsa than the shootings should.
"This community will not be defined by the treacherous, evil crimes of two individuals," he said. "That's not what Tulsa, Oklahoma, is about, and that's not what our people are about," he said. "The fact that this community drew together as one to stop this threat is what Tulsa is all about."
CNN's Maria P. White and Jason Carroll contributed to this report.
