UN and Arab League special envoy Kofi Annan has described the escalation in the violence in Syria as "unacceptable" and called for the government to abide by its promises to halt the violence.
An agreed April 10 deadline for a ceasefire has been thrown into fresh doubt after the regime demanded "written guarantees" that the rebels would halt their attacks ahead of any withdrawal of government troops.
The Syrian foreign ministry said Mr Annan had failed to provide written agreements for the "armed terrorist groups to stop the violence in all its forms".
The commander of the Free Syrian Army, Colonel Riad al Assad, responded by saying that his group would not provide guarantees in writing as the group does not recognise the government of President Bashar al Assad .
Col al Assad said if the regime withdraws its forces his fighters will comply with the ceasefire.
The Syrian government had agreed to withdraw its forces from populated areas by Tuesday under a six-point peace plan brokered by Mr Annan.
The rebel fighters were meant to abide by the ceasefire within 48 hours of the withdrawal.
Syria's implementation of the deal had already been in doubt after a sharp increase in the violence in several towns and cities which has left more than 100 dead in the past 24 hours.
"I am shocked by the recent reports of a surge in the violence and atrocities in several towns and villages resulting in an alarming level of casualties," Mr Annan said in a statement.
Pope Benedict XVI used his Easter Sunday message in Rome to appeal to the Syrian regime to end the violence.
"May there be an end to bloodshed and an immediate commitment to the path of respect, dialogue and reconciliation, as called for by the international community," he said.
Syrian activists say scores of civilians have been killed in assaults by government forces in Idlib province, close to the border with Turkey. Shelling has also continued in the cities of Homs and Hama.
Residents in the town of Latamna, in the province of Hama, claimed 40 civilians had died in what they described as a "massacre".
They said government forces had bombarded the town with artillery and gunfire.
Videos posted online showed rows of bodies, shrouded in white cloth, lined up for burial.
Another video, said to have been filmed in Homs, showed the bloodied corpses of at least a dozen young men who activists said had been executed.
Some appeared to have their hands bound.
Syrian activists claim the deal is another ruse by Mr Assad, who has flouted previous agreements to halt the violence that has claimed at least 9,000 lives over the past year.
Western nations are also sceptical about the regime's intentions.
The UN Security Council has endorsed a statement that includes the possibility of "further measures" being considered if the government does not implement the ceasefire.
But Russia and China have refused to back any punitive action - including sanctions - against the Syrian regime.
At least 2,800 Syrians have fled across the border into Turkey in the past few days, swelling the number of refugees living the camps to 24,000.
The UN's refugee agency says it is ready to offer assistance to the Turkish government to deal with the influx.