The United Nations and Arab League envoy to Syria says he has received assurances from Damascus that it will respect his ceasefire plan.
Kofi Annan made the comments during a visit to Iran on Wednesday and 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, the BBC reports.
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.
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."
However, he said the Syrian government 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".
Give peace a chance - NATO
The Supreme Commander of NATO in Europe says peace must be given a chance in Syria before military intervention is considered.
Admiral James Stavridis is in New Zealand for talks with government officials.
He told Radio New Zealand's Morning Report on Wednesday that the United Nations, the Arab League and Kofi Annan are working hard to achieve peace.