Syria on Sunday demanded written guarantees 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.
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.
UN-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 emphasising 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 criticising 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.
No comment on the fighting was immediately available from Syrian officials. The government has placed tight restrictions on media access in Syria, making it hard to verify witness accounts.