Just hours after UN monitors suspended their work, President Bashar al-Assad's army intensified shelling of Sunni Muslim regions in Syria, killing at least 50 people and wounding hundreds, opposition activists said.

The monitors' cessation was the clearest sign yet that a peace plan brokered by international mediator Kofi Annan had collapsed after repeated violations by Assad's forces and rebels backing a Sunni-led revolt.

US President Barack Obama will hold talks with Russian President Vladimir Putin on the sidelines of a G20 summit in Mexico today, but expectations are low that they will break a deadlock over Syria's conflict.

Russia and China, veto-wielding members of the UN Security Council, have shielded Assad from Western-sponsored action beyond verbal condemnation of the violence.

Assad's foes say this stance gives the Syrian president a free hand to pursue his 15-month-old crackdown against protesters.

"Around 85 percent of Homs is now under shelling or bombardment with mortar rounds and heavy machineguns," opposition activist Abu Imad told Reuters.

Opposition sources said at least 11 people had been killed.

"Dozens of wounded are without treatment because all the hospitals have fallen under the control of shabbiha (ghosts)," said Imad, referring to militiamen loyal to Assad.

"The dead are the lucky ones."

Another activist, Mohammad al-Homsi, said: "Since the (UN) observers stopped working yesterday, we have seen a clear escalation."

On Saturday, chief UN monitor General Robert Mood said increased violence had forced his observers to suspend operations to oversee Annan's widely ignored April 12 ceasefire.

The Norwegian blamed both Assad's forces and rebels.

No independent verification was available on the opposition accounts of the violence on Sunday.

Homs, which had a population of one million at the start of the revolt, has been under constant army shelling since March when Assad's forces overran an opposition neighbourhood whose residents were among the first to take up arms.

Free Syrian Army rebels are holed up with civilians still in Homs after hundreds of thousands fled over the last year.

DAMASCUS ATTACK

Opposition sources said Assad's forces also stepped up use of heavy weaponry in areas on the edge of Damascus that have been at the forefront of the uprising.

Elite Republican Guard troops struck the Damascus suburb of Douma with tanks and rockets, killing six people and wounding 75 in an assault to try and re-establish control, the sources said.

"Several tanks advanced on Shukri al-Qouatli street... The Free Syrian Army is trying to hold them back," one activist, who would give his name only as Mohammad, said.

Among other incidents, 20 people were killed when the army used heavy artillery fire on rebellious towns and villages in the province of Aleppo bordering Turkey and in the neighbouring provinces of Idlib and Hama, opposition sources said.

In the city of Aleppo itself, businesses staged a strike to protest against Assad's crackdown and many shops closed, the sources said.

A rise in violence over the last month, including two massacres that cost the lives of 200 Sunni men, women and children in villages near Homs and another north-western city Hama, has prompted greater international condemnation of Assad.

The opposition is increasingly accusing Assad of waging a military campaign of ethnic cleansing in Homs to empty the city and surrounding countryside of the majority Sunni inhabitants.

Assad has repeatedly said he was resisting what he described as a foreign conspiracy to divide Syria that left him no option but to use force against "terrorists".

He is from Syria's Alawite sect, an offshoot of Shi'ite Islam, and has been shielded or backed by Russia, a Soviet-era ally of his father, and Iran, which regards Syria as the supply line for its proxy Hezbollah (Party of God) in Lebanon.

The UN says Syrian forces have killed 10,000 people in the crackdown on protest against Assad's rule that broke out in March last year, inspired by uprisings across the Arab world which have toppled four autocratic leaders.

Assad's government says foreign-backed Islamist militants have killed at least 2600 Syrian police and troops.

Washington has said it was consulting other world powers on "next steps" over Syria, but acknowledges a Libya-style military intervention to help topple Assad would be difficult and could destroy the country's myriad ethnic and sectarian mosaic.

China has already signalled misgivings about a French proposal to enforce Annan's peace plan for Syria, saying it opposed any approach "leaning towards sanctions and pressure".

- Reuters