The UN Security Council demanded Wednesday that Syria immediately implement a peace plan by special envoy Kofi Annan, even as government forces pounded rebel zones around Damascus and Homs.

Russia and China, which have blocked two resolutions on Syria, backed a western-drafted statement that called on President Bashar al-Assad to work toward a cessation of hostilities and a democratic transition. The council gave a veiled warning of future international action.

The statement, which carries less weight than a formal resolution, gives strong backing to a six-point plan that Annan, the UN-Arab League envoy, put to Assad during talks in Damascus this month.

As the statement was read out, shells from Syrian troops rained down on the Homs district of Khaldiyeh.

At least 22 people have been killed in two days of bombardments in Homs, and another 23 died elsewhere in the country on Wednesday alone, activists said.

Thirty-nine bodies were found in the Rifai sector of Homs, activists added. They had probably been killed at the same time as 48 women and children whose mutilated corpses were found on March 12.

There were also fierce clashes between rebels and security forces near an intelligence post in the Damascus suburb of Harasta, according to the Syrian Observatory for Human Rights.

The UN says well over 8,000 people have been killed in Syria in the past year.

The Security Council statement called on Assad and the opposition to work with Annan "towards a peaceful settlement of the Syrian crisis and to implement fully and immediately his initial six-point proposal."

It said Annan should regularly update the council on his efforts. "In the light of these reports, the Security Council will consider further steps as appropriate," the statement added.

The council gave "full support" to Annan's efforts to set up a Syrian-led transition to a "democratic, plural political system."

Annan has a team of advisors in Damascus determining whether it is possible to take the peace process forward.

The former UN chief was "encouraged by the united support of the Security Council behind his efforts and urges the Syrian authorities to respond positively," his spokesman Ahmad Fawzi said in statement.

UN Secretary-General Ban Ki-moon was also "encouraged by the clear and unified message" in the statement, said his spokesman Martin Nesirky.

Ban hoped "that this united action by the council on Syria can mark a turning point in the international community's response to this crisis."

Annan's plan calls for Assad to pull troops and heavy weapons out of protest cities, a daily two-hour humanitarian pause to hostilities, access to all areas affected by the fighting, and a UN-supervised halt to all clashes.

The mention of a political transition, along with Russia and China's backing for the statement, was a strong signal to the increasingly isolated Assad government, diplomats said.

"This sends precisely the strong and united message to the Syrian government and all other actors in Syria that they need to respond and respond quickly and immediately to the six-point plan," said Britain's UN ambassador Mark Lyall Grant.

In a sign of the new diplomatic moves over Syria, the Security Council also agreed on a press statement, proposed by Russia, that "condemned in the strongest terms" bomb attacks in Damascus and Aleppo over the weekend.

US Secretary of State Hillary Clinton praised the UN statement and warned Assad to carry out the peace plan.

"To President Assad and his regime, we say, along with the rest of the international community, take this path, commit to it, or face increasing pressure and isolation," Clinton told reporters in Washington.

Beijing urges Syria to "actively support and coordinate the good office of Annan, cease violence immediately, reflect political will and launch political dialogue as soon as possible in order to achieve an early political settlement of the Syrian crisis," China's UN Ambassador Li Baodong told the Xinhua state news agency.

The Russian and Chinese vetoes have left the Security Council in deadlock in recent months. But Russia's Foreign Minister Sergei Lavrov welcomed the Security Council's move.

"The document does not contain any ultimatums, threats or assertions about who is guilty," said Lavrov. Russia is Syria's main ally but it has indicated growing impatience with Damascus.

US Senator John McCain meanwhile Wednesday told AFP that Assad's "massacres of his own people" are putting pressure on European and American leaders to take more active roles in ending the crisis.

The rebels, he said, "deserve our assistance and international assistance to fight back."

European countries still want to press for a full, binding Security Council resolution on the crisis in Syria, however.

French envoy Gerard Araud called the statement "a small step by the Security Council in the right direction."

"A resolution is still on the table and we hope we will manage to obtain a Security Council resolution," he told reporters.