Speaking during a visit to Iran, Mr Annan said there could be "improved conditions on the ground" by Thursday morning, if all sides did so.
A deadline for Syria to withdraw troops and weaponry from population centres expires on Thursday.
But there has so far been little sign of a let-up in violence.
At least 100 people were reported to have been killed on Tuesday, with fresh shelling in Homs and military activity in other cities overnight and into Wednesday morning.
There were also reports of shooting across the border with Turkey, with bullets landing in a refugee camp.
Mr Annan said he had received "further clarifications" from the government of Bashar al-Assad on how they intended to suspend hostilities and respect the six-point peace plan.
"If everyone respects it I think by six in the morning on Thursday we shall see improved conditions on the ground," he said.
But he said Damascus was still seeking assurances that opposition forces would also stop the fighting "so that we could see cessation of all the violence".
Mr Annan was speaking in Tehran after talks with Iranian Foreign Minister Ali Akbar Salehi, during which he appealed for Iran's support.
He said the region "cannot afford another shock" and warned that any miscalculation or mistakes in Syria could have "unimaginable consequences".
Iran has been a key ally of Damascus, but Mr Salehi said that "as long as the peace plan continues its approach, Iran will support it".
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