DAMASCUS — Syria's president decreed a vote this month on a new constitution that would effectively end nearly 50 years of single party rule, state media said, as troops reportedly stormed centres of dissent.

A day after flatly rejecting UN allegations of crimes against humanity, Bashar al-Assad called the ballot for February 26, in a move clearly aimed at placating growing global outrage over the bloodshed.

Under the newly proposed charter, freedom is "a sacred right" and "the people will govern the people" in a multi-party democratic system, state television said.

The new constitution also states that Islamic case law will be used as a source for legislation, that the president must be Muslim and over the age of 40.

It also prohibits parties based on religion.

Assad, who in April lifted a state of emergency in force since 1963, when his Baath Party took power, has made repeated promises of reforms that have failed to materialise since a popular uprising erupted on March 15.

The embattled president, who succeeded his late father Hafez in 2000, has said the constitution would usher in a "new era" for Syria, SANA state news agency reported.

There was no immediate reaction to the announcement from Syria's political opposition, but analysts said it fell short of what was required to end the Arab Spring-inspired uprising.

"It's good that the regime at any point talks about reform or fundamental change, but... holding a referendum amid a civil war between the government and its people is unacceptable," said Paul Salem of the Carnegie Middle East Centre.

Assad's latest move came as activists said troops stormed the central city of Hama and stepped up their assault on protest hubs nationwide, defiant in the face of mounting Arab and Western efforts to end the bloodshed.

Further to the south, an explosion struck an oil pipeline at daybreak in the flashpoint city of Homs, with activists saying government forces bombed it from the air and state media blaming "armed terrorist gangs."

Syria's government on Tuesday rejected UN charges of crimes against humanity, as monitors accused Assad's forces of launching one of their heaviest assaults yet in a 12-day onslaught on Homs.

And after Russia and China twice vetoed a resolution calling on Assad to stop the deadly attacks on civilians, diplomats said they would now seek a condemnation of the violence at the UN General Assembly on Thursday.

Rights groups say more than 6,000 people have been killed since government forces launched a brutal crackdown on protests calling for democracy, which were launched 11 months ago to the day.

The pipeline blast in Homs sent up columns of black smoke over the central city, according to footage uploaded by opposition activists to video-sharing website YouTube.

Hadi Abdullah of the opposition General Commission of the Syrian Revolution said it was the third such attack on the same pipeline, although this was the first time that it was targeted with aircraft.

The reported use of warplanes could not be immediately verified.

Syria's government has attributed several similar attacks to foreign-backed "terrorists", but the opposition accuses it of destroying energy infrastructure to punish dissenters.

Assad's forces appear to have refrained from using air power to crush armed rebels to avoid a no-fly zone being imposed over the country.

On the ground, however, troops launched an assault on the city of Hama, just north of Homs, where loud blasts could be heard in the Hamidiyeh and other neighbourhoods, said the Syrian Observatory for Human Rights.

Rami Abdel Rahman, head of the Britain-based monitoring group, said ongoing clashes had killed 20 people, including nine civilians, in Al-Atareb, northwestern Aleppo province.

A bomb blast killed three people in Jisr al-Shughur, a 16-year-old schoolgirl was shot dead in southern Daraa province and gunfire and explosions rocked the northern city of Idlib, said the Observatory.

In Damascus, dozens of young Syrians gathered in front of the Russian embassy to thank Moscow and Beijing for their support, state television reported.

"We are here to thank Russia and China," said one woman who urged Assad to deploy the army across Syria and to strike what she called gangs "with an iron fist."

As the diplomatic gears turned, China said it wanted an immediate halt to the spiralling bloodshed in Syria and an "inclusive dialogue" between the government and opposition protesters.

China and Russia have faced a barrage of criticism for blocking a UN Security Council resolution condemning the bloody crackdown on protests in Syria, including from Arab nations with which Beijing normally has good ties.