The peace plan proposed by the UN and Arab League envoy to Syria, Kofi Annan, expects the Syrian Government to implement the peace plan immediately.
The plan includes the need for an UN-supervised cessation of armed hostilities by all parties.
However, reports by activists indicate the Syrian regime is continuing its offensive against rebel forces and shelling the city of Homs.
Ahmed Fawzi, spokesperson for Mr Annan, told reporters in Geneva:
“This is our great concern. We expect [President Bashar al-Assad] to implement this plan immediately. The deadline is now.
"If you read the agreement... it specifically asks the government to withdraw its troops, to cease using heavy weapons in populated centres.
"The very clear implication here is that the government must stop first and then discuss a cessation of hostilities with the other side and with the mediator.
"The rationale is very simple. We are appealing to the stronger party to make a gesture of good faith and stop the killing. We are certain that if that happens, the opposition will follow suit."
Syria accepted the plan on Tuesday but President Assad said it was "necessary to obtain commitments from other parties to halt the terrorist acts by the armed groups and to withdraw the weapons of these groups and call on them to stop their terrorist acts".
Countries which "support the armed groups with money and weapons must be persuaded to stop this immediately", he added.
