Syrian government forces attacked the opposition strongholds of Homs and Hama on Wednesday and also raided a district of Damascus in the closest military operation to the capital's centre since an uprising against President Bashar al-Assad broke out 11 months ago.

Elite forces backed by armoured personnel carriers erected roadblocks in main streets of Damascus' residential Barzeh neighbourhood, searched houses and made arrests, witnesses said.

Residents said they were looking for opposition activists and members of the rebel Free Syrian Army, which has provided armed protection for protests against Assad in the district.

Government forces also mounted an offensive in Hama, a city with a bloody history of resistance to the 42-year Assad dynasty, firing on residential neighbourhoods from armoured vehicles and anti-aircraft guns, opposition activists said.

Artillery also shelled Sunni Muslim neighbourhoods in Homs, the 13th day of their bombardment of a city that has been at the forefront of the uprising.

The attacks indicated Assad's determination to crush the revolt despite international efforts, spearheaded by Arab nations, to make him cease the bloodshed.

French Foreign Minister Alain Juppe said France had started negotiating a new U.N. Security Council resolution on Syria with Russia and wanted to discuss creating humanitarian corridors to ease the plight of civilians caught up in the violence.

The idea of humanitarian corridors that I previously proposed to allow NGOs to reach the zones where there are scandalous massacres should be discussed at the Security Council, Juppe told France Info radio.

The U.N. General Assembly would vote on a symbolic resolution on Thursday that would add to pressure on the Assad government, he said.

Russia, Syria's long-time ally and main arms supplier, and China vetoed a U.N. Security Council resolution on February4 that would have called on Assad to step down.

Since then Arab nations led by Gulf state rivals to Assad have been working to put a new resolution before the General Assembly. Unlike a Security Council resolution, it cannot be vetoed but would not be binding.

The Arab League has also called for a joint U.N.-Arab peacekeeping force to be deployed in Syria and indicated members would be willing to arm the opposition -- moves that have caused concern among Western powers eager to see the end of Assad's 11-year-rule but wary of a foreign military intervention which could spark a wider regional war.

Adding to the regional pressure on Assad, the head of Egypt's influential seat of Sunni Islamic learning, al-Azhar, called for bold Arab action against the Syrian government.

In Washington, U.S. President Barack Obama told Chinese Vice President Xi Jinping at a meeting at the White House on Tuesday that the United States was disappointed with China's veto of the Security Council resolution, a U.S. official said.

Chinese Vice Foreign Minister Cui Tiankai said after the talks that China still supported the role of the Arab League and wanted a dialogue to end the violence. But the Security Council needed to take a very careful and very responsible attitude to Syria, he said.

If the U.N. Security Council takes the wrong steps, that could lead to even worse bloodshed, Cui said.

ON HIS OWN TERMS

Assad dismisses his opponents as terrorists backed by enemy nations in a regional power-play and says he will introduce political reforms on his own terms.

Syrian state television reported on Wednesday he had ordered a referendum on a new constitution on February 26.

That move is unlikely to appease the movement against him, which began with street demonstrations by civilians but now includes armed insurrection by the Free Syrian Army made up mainly of army defectors.

The government says at least 2,000 members of its military and security forces have died and the United Nations says government forces have killed several thousand civilians.

On the conflict front, tanks deployed near the citadel of Hama were shelling the neighbourhoods of Faraya, Olailat, Bashoura and al-Hamidiya, and troops were advancing from the airport, opposition sources said.

An activist called Amer, speaking briefly by satellite phone, said landlines and mobile phone networks have been cut in all of Hama, a Sunni city notorious for the massacre of about 10,000 people when the present president's father Hafez sent in troops to crush an uprising there in 1982.

Activists said no casualty reports were available from Hama, Syria's fourth largest city, because of communications problems.

Foreign media must rely largely on often-unverified activist accounts because the Syrian government restricts access.

In the Damascus operation, witnesses said at least 1,000 soldiers had swamped Barzeh district after sealing off roads.

They have destroyed the facades of shops and turned back students heading to school, Mazen, a university student, said by telephone from Barzeh.

The largely Sunni Muslim district has been among the most active in the capital in the protest movement against Assad, whose family are from the minority Alawite sect.

In Homs, an explosion hit an oil pipeline feeding a refinery, sending a large plume of smoke rising into the sky, witnesses said. The blast hit the pipeline near a district being shelled by government troops, witnesses said.

Government forces have bombarded pro-opposition areas of Homs for 13 days, killing hundreds and drawing international condemnation.

Activists and aid groups report a growing humanitarian crisis there, with food running short and wounded people unable to receive proper treatment.

France said it had created a one million euro emergency fund for aid agencies looking to help the Syrian people and would propose a similar one at an international level next week at a meeting in Tunisia to discuss the crisis.

It also renewed the idea of creating humanitarian corridors, first suggested in November, with Syrian approval or with an international mandate for shipping food and medicine to alleviate civilian suffering.

Under that plan, the corridors would link Syrian population centres to the frontiers of Turkey and Lebanon, to the Mediterranean coast or to an airport.

(Additional reporting by Dominic Evans and Erika Solomon in Beirut, and John Irish in Paris, Writing by Angus MacSwan; Editing by Peter Millership)