A peace plan for Syria was in jeopardy after the regime laid down conditions for a pullout of government troops from protest hubs and fresh clashes raged on Monday.

In latest fighting, 12 soldiers were killed in fierce fighting between the army and rebels in the Aleppo region of northern Syria and Deir Ezzor in the east, the Syrian Observatory for Human Rights said.

It also reported army operations in several other parts of the country.

Under a peace deal brokered by Kofi Annan, the former UN chief, the Syrian army was scheduled to withdraw from protest cities on Tuesday, with a complete end to fighting set for 48 hours later.

But the Damascus regime has since said it would only carry out its side of the bargain if rebels first handed over written guarantees to stop fighting.

Rebel army chief Colonel Riyadh al-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."

The regime demand came as a weekend escalation in violence claimed almost 180 lives, most of them civilians, a surge in bloodshed that Annan described as "unacceptable."

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 ahead of the April 10 deadline.

In a statement in Geneva, Annan urged the 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."

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.

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 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 travel 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.

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 President Bashar al-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 Syrian security forces and pro-regime militias had executed more than 100 civilians and rebel fighters in attacks on protest hubs since late 2011.

Many of the summary and extrajudicial executions were carried out last month, and at least 85 of those killed were residents who never took part in fighting, including women and children, said the New York-based watchdog.