Syria's government is demanding written guarantees from opposition leaders before it agrees to withdraw its own forces and respect a ceasefire.
Damascus issued its two new demands just 48 hours before the deadline for the Syrian army to withdraw from towns and cities, the BBC reports.
A UN-brokered plan requires the government of President Bashar al-Assad to begin pulling back troops by 10 April.
The rebels said they had already given their word to stop fighting if government forces did so too.
An opposition body, the Syrian National Council, said it believed the government's new demands were a ploy by President Assad to reject the peace plan.
The opposition group says it is prepared to sign a guarantee that it will stop fighting and disarm, though it isn't necessary.
As many as 160 people were killed on Saturday, many of them said to be civilian victims of shelling by government forces, and fierce new fighting was reported on Sunday.