International envoy Kofi Annan has insisted the Syrian government will meet the deadline for a ceasefire.
Annan, acting as special envoy for the United Nations and the Arab League, said the government had given him assurances they would keep their side of the ceasefire deal.
"If everyone respects it I think by six in the morning on Thursday we shall see improved conditions on the ground," Annan said after meeting Iranian foreign minister Ali Akbar Salehi in Tehran.
Annan added that Iran and other regional countries had to be part of the solution.
"The political process must be Syrian-led and respect the aspirations of the Syrian people," Annan said. "What is important is that governments in the region and beyond work with Syria to resolve the crisis.
"The geopolitical position of Syria is such that any miscalculation can have unimaginable consequences."
Also on Wednesday China said it was deeply concerned about the situation inside Syria.
"A political solution to the Syrian issue has reached a critical stage," said Chinese foreign ministry spokesman Liu Weimin. "Violence within Syria continues and civilian casualties are rising. China expresses its deep worries."
Foreign governments, including the UK, and the Syrian opposition remain sceptical about the chance for a meaningful cessation in the violence after evidence emerged of fresh attacks by government soldiers on residential areas.
At least 100 people were killed on Tuesday after shelling in Homs and other cities.
The Syrian Network for Human Rights said that 128 civilians were killed, including more than 70 in Homs alone.
Activists also released video that they said showed evidence of shelling in Homs on Wednesday, less than 24 hours before the start of the mooted ceasefire:
Elsewhere CNN said that a Turkish refugee camp was hit by bullets fired by the Syrian army. It showed images from Turkish media of automatic gunfire from a government surveillance building directed at Syrians trying to escape over the border. Earlier this week two people were reportedly killed in a similar incident.
More than 1,000 people have been killed in Syria in little over a week, activists said. The UN estimates more than 9,000 people have been killed since the start of the uprising in March 2011.