Syrian government forces attacked the opposition strongholds of Homs and Hama and also raided a district of Damascus in the closest military operation to the capital's center 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 neighborhoods 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.