A UN-brokered peace deal for Syria appeared to collapse as the government made a new demand that its opponents provide "written guarantees" to lay down their weapons before regime forces withdraw from cities, a call swiftly rejected by the country's main rebel group.
The deal, brokered by UN and Arab League envoy Kofi Annan, was due to take effect on Tuesday, paving the way for negotiations to end the country's year-old crisis, which the UN estimates has killed 9,000 people.
Mr Annan said last week that Syrian president Bashar Assad had accepted the plan and its call for government forces to pull back from urban centres.
But on Sunday Syria's Foreign Ministry spokesman, Jihad Makdessi, placed a new condition - that the opposition agree in writing "to halt violence with all its forms and their readiness to lay down weapons."
The commander of the rebel Free Syrian Army, Riad al-Asaad, said his group was prepared to abide by the Annan agreement, but rejected the government's new unilateral demand.
The FSA does not recognise the regime "and for that reason we will not give guarantees," he said, adding the government should withdraw its forces to bases and remove checkpoints from streets.
Mr Annan's deal calls for government forces to withdraw from population centres on Tuesday, to be followed by a full ceasefire by both sides by 6am on Thursday.
However, in recent days, Syrian forces have stepped up shelling attacks on restive towns, and activists say scores of civilians have been killed daily - at least 100 on Saturday alone.
Mr Annan said in a statement that "the present escalation of violence is unacceptable".
He also reminded the government "of the need for full implementation of its commitments," though it was unclear whether his statement was issued before or after Damascus imposed new conditions for complying with the truce.