A peace plan for Syria was in jeopardy on Monday as fresh clashes raged after President Bashar al-Assad's government laid down conditions for it to pull troops and armour out of protest hubs.

Under a peace deal brokered by UN and Arab League envoy Kofi Annan, Syria's armed forces are supposed to withdraw from protest centres on Tuesday, with a complete end to fighting set for 48 hours later.

But the truce already appears in jeopardy after Damascus said it would only carry its side of the bargain if rebels first handed over written guarantees to stop fighting, a demanded rejected by rebel army chief Colonel Riyadh al-Asaad.

The 11th-hour demand came as weekend violence claimed almost 180 lives, most of them civilians, a surge in bloodshed that former UN chief Annan described as 'unacceptable.'

Making matters worse, fresh fighting killed another 13 people on Monday, all but one of them soldiers, according to the Syrian Observatory for Human Rights.

"The regime had thought that it would control all areas (of rebels by April 10). As this is not happening, it is procrastinating to gain time," said the Observatory's Rami Abdel Rahman.

"If the Annan plan does not work, no other plan would, and Syria would plunge into a civil war," he said.

The Observatory also reported army operations in several other parts of the country, and the killing of a civilian in ambush near the village of Saida, in southern Daraa province.

Two Syrians also died of wounds after fleeing to a refugee camp in Turkey following a shootout with Assad loyalists at Salama village in the northern Syrian province of Aleppo, Anatolia news agency said, citing a health official.

Diplomatic sources in Ankara said the two died in shooting from the Syrian side of the border into Turkey, and that a Turkish translator was wounded.

Fifteen others were wounded in the shootout, but it was not clear if they were civilians or rebel fighters.

Around 25,000 Syrian refugees are currently housed in camps in Turkey's three provinces bordering Syria, where civilians have been fleeing the deadly crackdown over the past year.

The Milliyet newspaper reported Monday that Turkey would consider using troops to secure humanitarian corridors in border areas should the number of Syrian refugees swell to above 50,000.

Chinese foreign ministry spokesman Liu Weimin, meanwhile, urged the Syrian government to honour its truce commitments.

"China urges the Syrian government and parties concerned in Syria to seize the important opportunities, to honour their commitment of ceasefire and withdrawal of troops," said Liu.

Syrian Foreign Minister Walid Muallem was scheduled to fly on Monday to ally Moscow, which along with Beijing has blocked two UN Security Council draft resolutions condemning Damascus for its bloody crackdown.

On Sunday, the Syrian foreign ministry outlined the regime's new conditions in a statement.

"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," it said.

It said the regime was also awaiting written guarantees from the governments of Qatar, Saudi Arabia and Turkey "on stopping their funding to terrorist groups," referring to the regime's key regional critics.

Rebel army chief Colonel Asaad countered: "We are committed to the Annan plan... We will present our guarantees and our commitments to the international community, but not to this (Syrian) regime."

FSA spokesman Colonel Kassem Saadeddine reiterated on Monday the rebel force's readiness to cease fire on Tuesday "if the regime commits to respecting the terms of the (UN) plan."

Annan is to travel to Turkey on Tuesday, on the eve of a trip to Syria's ally Iran, for a visit to Syrian refugee camps near the border, a Turkish diplomatic source said.

On Sunday, he urged Syria's government to respect its commitments on troop withdrawals.

"I remind the Syrian government of the need for full implementation of its commitments and stress that the present escalation of violence is unacceptable."

The Security Council has formally endorsed the Tuesday deadline for a ceasefire, but Damascus said a day later that the number of "terrorist acts" has risen since the deal was agreed with UN-Arab League envoy Annan.

Annan "said he would work to stop the violence, disarm armed groups... initiate a comprehensive national dialogue with opposition movements," when he met Assad last month, the foreign ministry said.

France denounced as 'unacceptable' the new Syrian demands.

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.

On Monday, Human Rights Watch said security forces and pro-regime militias had executed more than 100 civilians and rebels in attacks on protest hubs since late 2011, urging any UN mission to Syria to collect evidence.