BEIRUT: Army deserters killed 27 soldiers in southern Syria on Thursday, an activist group said, in some of the deadliest attacks on forces loyal to President Bashar al-Assad since the start of an uprising nine months ago.

The Syrian Observatory for Human Rights said the clashes flared in the southern city of Deraa, where protests against Assad first erupted in March, and at a checkpoint east of the city where all 15 personnel manning it were killed.

It did not say how they broke out, but the high casualties among security forces suggested coordinated strikes by the army rebels who have escalated attacks in recent weeks, raising the spectre of Syria slipping towards civil war.

The United Nations says 5,000 people have been killed in Assad's crackdown on protests inspired by uprisings elsewhere in the Arab world. Assad has denied any orders were issued to kill demonstrators and says gunmen have killed 1,100 of his forces.

But a report published by Human Rights Watch on Thursday, based on interviews with dozens of defectors, said army commanders have ordered troops to use "all means necessary" to halt protests, often giving explicit instructions to open fire.

One special forces soldier said his brigade was told to "use as many bullets as you want" on protesters in Deraa in April.

A sniper in the city of Homs said his commanders ordered that a specific percentage of demonstrators should die. "For 5,000 protesters, for example, the target would be 15 to 20 people," he told Human Rights Watch (HRW).

HRW identified 74 commanders who ordered, authorised or condoned killings, torture and unlawful arrests during the anti-government protests. "These abuses constitute crimes against humanity," it said, calling on the United Nations Security Council to refer Syria to the International Criminal Court.

Assad, 46, whose family is from the minority Alawite sect, has held power in majority Sunni Muslim Syria for four decades, is facing the most serious challenge to his 11-year rule.

Rami Abdulrahman of the British-based Observatory said a large battle on Thursday began around dawn in two districts of Deraa city and a joint army-security checkpoint near Musayfrah, about 25 km (15 miles) east of Deraa.

Army rebels have stepped up their campaign against security forces in the last month, ambushing military convoys, opening fire on an intelligence centre on the outskirts of Damascus and killing six pilots at an air base.

The bloodshed prompted the head of the main Syrian opposition group to call on the rebels forces' Free Syrian Army to restrict operations against Assad's military to defending protests. "We want to avoid a civil war at all costs," Burhan Ghalioun told Reuters last week.

But his influence over the insurgents appears limited.

One Free Syrian Army officer, speaking before Thursday's clashes, said the rebels were justified in targeting Assad's forces and said Ghalioun's comments betrayed "a lack of knowledge of the military basis of this regime".

"Anyone who bears arms against civilians, either army, security or shabbiha (pro-Assad militia), and kills civilians -- we will respond and inflict whatever damage we can," said Major Maher Ismail al-Naimi.

"In our view, that does not mean the revolution is abandoning its peaceful nature," said Naimi, who deserted from the Republican Guards. He is now the spokesman for the Free Syrian Army but said he was giving his personal opinion.

"This is a defensive strategy against a regime which has deployed special forces across the country, artillery, tanks and machine guns to kill civilians and defectors."

The United States and France, which blame Assad's forces for the violence, have urged the U.N. Security Council to respond to the mounting death toll.

"This cannot go on," U.N. Secretary-General Ban Ki-moon told reporters in New York. "In the name of humanity, it is time for the international community to act."

But Syria's foreign backers Russia and China have blocked Western efforts to secure Council condemnation of Damascus. Its closest regional ally Iran promised support for Syria's economy, ravaged by the unrest, Western sanctions and an Arab League decision to halt business with the central bank.

On Wednesday, Syrian troops swept into the city of Hama to break a "Strike for Dignity" called by the opposition, killing at least 10 people but running into resistance from insurgents who destroyed two armoured vehicles, activists said.

The assault in Hama was the first armoured incursion there since a tank offensive in August crushed huge protests in the city. Activists said troops fired machineguns and ransacked and burnt shops which had closed to observe the opposition's mass, open-ended strike.

Despite the worsening economic crisis and a growing number of army defections, mainly among Sunni conscripts, Assad still has the loyalty of most of the army. Unlike in Libya, the rebels have secured neither high-level defections from the military or government, nor do they fully control any territory.