The UN and Arab League envoy to Syria, Kofi Annan, expects the government to implement his peace plan immediately.
Mr Annan's spokesman, Ahmed Fawzi, told reporters: "The deadline is now."
The peace plan, which the government accepted on Tuesday, calls for a UN-supervised cessation of armed violence in all its forms by all parties.
Earlier, activists said government forces had clashed with armed rebels in the north-western province of Idlib and shelled the central city of Homs.
Two people were killed near the village of Bsas, in Homs province, when their car was fired at, the Syrian Observatory for Human Rights said.
At least 20 people were killed in clashes across Syria on Thursday.
'No hope of survival'
Mr Fawzi said there clearly not been a "cessation of hostilities on the ground" in Syria this week despite the government's acceptance of Mr Annan's peace plan, which has the backing of the UN Security Council.
"This is out great concern," he told a news conference in Geneva.
Mr Annan wanted government forces to implement the ceasefire first, but also the rebels to "lay down their arms and start talking", Mr Fawzi added.
"We expect [President Bashar al-Assad] to implement this plan immediately."
In addition to a ceasefire, the peace plan calls for the withdrawal of soldiers and heavy weapons from cities, the release of prisoners, delivery of humanitarian aid for Syrians who need it, and free movement for journalists.
On Thursday, Arab leaders meeting in Baghdad called for Mr Annan's peace plan to be implemented immediately and completely.
President Assad, who was not invited, said he accepted the initiative, but that armed groups inside Syria had to cease "terrorist acts".
Meanwhile, the UK announced an extra £500,000 ($700,000) of support for Syrian opposition groups both inside and outside the country.
Foreign Secretary William Hague said the money would provide non-lethal support to political opponents of President Assad, including to help activists co-ordinate protests and gather evidence of atrocities.
He urged Mr Assad to accept he had no hope of political survival.