The Syrian government demanded written guarantees Sunday that insurgents will stop fighting before it pulls back troops under the terms of a peace plan, casting additional doubt over a truce due to start this week.
The rebels said they had already given their word to stop fighting if government forces did so too, the BBC reported.
The regime will not implement this plan. This plan will fail, Free Syrian Army chief Riad al-Asaad told Reuters.
Escalating violence has already raised questions over the ceasefire. Opposition activists said dozens of people were killed and wounded on Sunday when President Bashar al-Assad's loyalists shelled a rebellious area near the border with Turkey.
The opposition Syrian National Council said the government's new demands were a ploy by President Bashar al-Assad to reject the peace plan.
Escalating violence has already raised questions over the ceasefire. Opposition activists said dozens of people were killed and wounded on Sunday when Assad's loyalists shelled a rebellious area near the border with Turkey.
U.N.-Arab League envoy Kofi Annan, seeking to end the conflict that has killed more than 9,000 people in the past year, said the latest bloodshed violated the guarantees he had been given and urged Damascus to keep its promises.
The deal Annan brokered calls on Syria to begin the pullback of troops from around towns and cities by Tuesday for a truce to start 48 hours later.
While emphasizing that would happen, Foreign Ministry spokesman Jihad Makdissi said in a statement that Syria also wanted the written guarantees.
Syria has a plan for military pullback already in place and being implemented, but completing and achieving the main goal would definitely require the guarantees from the other side and those supporting them to abide by the terms of calm, he said.
Syria also sought guarantees that Qatar, Saudi Arabia and Turkey - outspoken in criticizing Assad - would not fund the armed groups.
Annan made no specific reference to the new Syrian demands in a statement from his office in Geneva.
He expressed shock at the surge in violence and atrocities. Each side has accused the other of intensifying assaults in the run-up to the truce.
As we get closer to the Tuesday 10 April deadline, I remind the Syrian government of the need for full implementation of its commitments and stress that the present escalation of violence is unacceptable, he said.
Dozens of people were killed and wounded when Assad's tanks shelled an area in the rebellious province of Idlib, near the border with Turkey, opposition activists said.
Fighters from the rebel Free Syrian Army were surrounded in the village of al-Bashiriya, activists said.
The army is shelling al-Rouge with tanks, and helicopters are firing rockets at al-Bashiriya. Tens of people have fallen dead or injured but we cannot get to them because the bombardment is heavy, said activist Mahmoud Ali, with the sound of helicopters audible on the phone.
A major Syrian army offensive to seize back large swathes of Idlib that had fallen under rebel control has killed and wounded hundreds of people in the last 10 days. Thousands of Syrians have fled to Turkey.
FSA leader Asaad said his group had not been asked to deliver written guarantees to end violence.
We have given our word that if the regime commits to the plan then we will too, he said. We are honest.
Nobody has asked us for anything written. Nobody has discussed with us handing over our weapons. We will never hand over our weapons.
Colonel Qassem Saad al-Deen, spokesman for the joint command of the FSA inside Syria, said the rebels would respect the deadline to cease fire.
We will commit to the deadline even if they (government forces) do not pull back, we will cease fire as we have pledged to the U.N., he said.
But if they fire we will pick up arms again and fight them, he told Reuters from inside Syria.
When the regime asks Kofi Annan for written guarantees that we will drop our weapons it is actually mocking the United Nations. This is a joke.
He said at least 1,000 people had been killed during last week's violence in Syria, most of them civilians.
The Syrian government says its opponents have killed more than 2,500 troops and police since the unrest began in March 2011.