Syrian troops yesterday attacked rebel strongholds in Deraa, where the uprising against President Bashar al-Assad erupted nearly a year ago.

The assault on Deraa followed a push against rebels in the cities of Hama and Homs. Homs has faced nearly two weeks of bombardment from Assad's forces in an apparent drive to crush the 11-month-old uprising against his rule.

Assad has intensified a crackdown on protesters and insurgents, while also setting a February 26 referendum on a draft constitution to end the Baath Party's monopoly on power, to be followed by a multiparty parliamentary election.

Syria's opposition and Western powers dismissed the promised reforms, and UN secretary-general Ban Ki-moon, speaking before a non-binding vote at the UN on a draft resolution aimed at increasing pressure on Assad, said potential crimes against humanity were being carried out in Syria.

"We see neighbourhoods shelled indiscriminately, hospitals used as torture centres, children as young as 10 years old killed and abused. We see almost ... certain crimes against humanity," he said.

China, which along with Russia blocked a draft resolution at the UN Security Council backing an Arab call for Assad to step aside, said it was sending a senior envoy to Syria.

"[China] does not approve of the use of force to interfere in Syria or the forceful pushing of a so-called regime change," Deputy Foreign Minister Zhai Jun said.

Zhai, who will be in Syria today and tomorrow, said China believed that "sanctions or the threat of sanctions are not conducive to the appropriate resolution of this issue".

An authoritative Chinese newspaper, apparently responding to criticism of the Chinese and Russian vetoes, said meddling in Syria by foreign powers risked stirring up a hornet's nest of bloodshed and instability in the region.

After bombarding Homs for nearly two weeks, the military has begun a new offensive in Hama, a city with a bloody history of resistance to Assad's late father, Hafez al-Assad. Activists said at least 14 people were killed in the bombardment of the nearby town of Kfar Nubouzeh yesterday.

The state news agency said security forces "chased and fought an armed terrorist group in the Hamidiya neighbourhood of Hama that has been [terrorising] citizens" and arrested some of its members.

In Deraa, a city on the Jordanian border, the sound of explosions and machine-gun fire echoed through districts under attack by government troops, residents said.

The Syrian Observatory for Human Rights said three members of the security forces were killed in clashes with army deserters. There was no immediate comment from Syrian authorities, who restrict media access to the country.