Annan received a letter from the Syrian minister of foreign affairs less than 24 hours before the agreed upon date to begin a cease-fire with opponents of the regime of President Bashar Assad.
The letter said government forces would "cease all military fighting throughout Syrian territory as of 6 a.m. [Damascus time] tomorrow, Thursday, 12 April 2012," Annan spokesman Ahmed Fawzi said.
However, the letter said Syria reserved "the right to respond proportionately to any attacks carried out by armed terrorist groups against civilians, government forces or public and private property."
The United Nations estimates more than 8,000 people, mostly civilians, have been killed and tens of thousands displaced since protests in Syria began in March 2011.
Fawzi said Annan, who visited Damascus last month, would continue to push the Syrian government and the opposition to implement his six-point plan to halt the violence. Annan is to address the U.N. Security Council on Syria Thursday.
"We still have time between now and 12 April to stop the violence," Annan said earlier after visiting Syrian refugee camps on the Turkish side of the border with Syria -- an experience he called "heart-wrenching."
Annan's six-point plan originally called for Syria to withdraw troops and weapons from populated areas by 6 a.m. local time Tuesday and for all hostilities against Syrian citizens to end by 6 a.m. Thursday.
Syrian Foreign Minister Walid Mualem said Tuesday Assad forces had withdrawn from part of the besieged western-central city of Homs.
The opposition Local Coordination Committees of Syria said most of the 101 people killed in fighting Tuesday were in Homs.
The Assad government said it would withdraw its troops and abide by a cease-fire only if the opposition fighters, whom it calls terrorists, put down their weapons and if Qatar, Saudi Arabia and Turkey guarantee they will "stop funding and arming terrorist groups."
Annan said Tuesday these requirements were not part of his original agreement with the Syrian regime.
Syrian ally Russia said Tuesday the Assad government "could have been more decisive" in implementing the plan. But Russian Foreign Minister Sergey Lavrov also called on opposition forces to halt violence.
The opposition, including the rebel Free Syrian Army, said it would comply with the truce if the regime does.