Syria on Sunday demanded guarantees that armed groups cease fire before withdrawing its troops from protest hubs as agreed with special envoy Kofi Annan, even as a UN truce deadline loomed.
The foreign ministry's request came as a weekend escalation in violence claimed almost 180 lives, most of them civilians, ahead of the UN deadline of Tuesday for regime forces to cease fire.
The surge in bloodshed prompted harsh words from Annan who said the escalation was "unacceptable" while France denounced as "unacceptable" Syria's demand for guarantees.
Monitors said 51 people were killed on Sunday, a day after nearly 130 died across Syria, while 200 others were arrested in raids by government forces pushing to crush dissent.
"To say that Syria will pull back its forces from towns on April 10 is inaccurate, Kofi Annan having not yet presented written guarantees on the acceptance by armed terrorist groups of a halt to all violence," the foreign ministry said in a statement.
On Thursday, the UN Security Council formally endorsed the deadline, but Damascus said a day later that the number of "terrorist acts" has risen since the deal was agreed with the UN and Arab League envoy.
"Mr Annan has not submitted written guarantees from the governments of Qatar, Saudi Arabia and Turkey on stopping their funding to terrorist groups," the foreign ministry said of its key regional critics.
Annan's peace plan calls for the withdrawal of the Syrian army from protest cities on Tuesday, with a complete end to fighting 48 hours later.
"Syria is not going to repeat what happened in the presence of Arab observers when armed forces left towns," the foreign ministry said, referring to a failed Arab monitoring mission at the turn of the year.
Annan "said he would work to stop the violence, disarm armed groups... initiate a comprehensive national dialogue with opposition movements," when he met President Bashar al-Assad last month, the ministry said.
"It is this principle on which Syria accepted Annan's mission and his six-point plan."
Reacting to the challenge from the regime, the chief of the Free Syrian Army, which is made up mostly of deserters from the military, said his rebel fighters will respect the peace plan.
"We are committed to the Annan plan," Colonel Riyadh al-Asaad told AFP in Beirut by telephone. "We will present our guarantees and our commitments to the international community, but not to this (Syrian) regime."
France denounced as "unacceptable" the fresh Syrian demands in a strongly worded foreign ministry statement.
Assad had already accepted the terms of international envoy Kofi Annan's peace plan, it said.
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The statement condemned what it called the "continuing massacres" and renewed French support for Annan.
In a statement in Geneva, Annan expressed "shock" at the surge in violence, calling the bloodshed "unacceptable" and urging the government in Damascus to respect its commitments.
"I remind the Syrian government of the need for full implementation of its commitments and stress that the present escalation of violence is unacceptable."
"This is a time when we must all urgently work towards a full cessation of hostilities, providing the space for humanitarian access and creating the conditions for a political process," Annan said.
The former UN chief said he was in "constant contact" with the government, and asked "all states with influence on the parties to use it now to ensure an end to the bloodshed and the beginning of dialogue."
Syrian Foreign Minister Walid Muallem is scheduled to travel to ally Moscow on Monday.
The United Nations says more than 9,000 people have been killed since anti-regime protests broke out in March 2011, while monitors put the number at more than 10,000.
The main umbrella opposition group the Syrian National Council called for UN intervention after monitoring groups said 86 of nearly 130 people killed on Saturday were civilians.
"We urge the UN Security Council to intervene urgently to stop the humanitarian crisis caused by the regime ... by adopting a resolution under Chapter 7 to ensure the protection of civilians," the SNC said.
Another 26 civilians, 17 soldiers and eight deserters were killed across the country on Sunday, monitors said.
Meanwhile the Syrian Observatory for Human Rights reported that deserters shot down a military helicopter in the northwest Idlib province, where fighting raged between rebels and army troops, quoting "several sources."
"The (unidentified) sources said they saw the aircraft exploding in the air before crashing," the Britain-based monitoring group said in a statement, adding that the incident was the first of its kind in the year-long conflict.
The incident, which could not be immediately confirmed by rebel or official sources, reportedly took place in the Jisr al-Shughur region of Idlib.
Belgium's Foreign Minister Didier Reynders said a humanitarian intervention under the protection of military forces could be the next stage if the Annan plan is not respected.
Pope Benedict XVI called in an Easter Day message for an end to the killings in Syria and a commitment to dialogue, as part of a wider appeal to give hope to the entire Middle East.