Syria pounded protest hubs Wednesday, despite mounting pressure from world powers, as peace envoy Kofi Annan said there is still a chance to salvage his plan to halt hostilities within hours.

Annan rejected calls to arm rebels, saying such a move would be "disastrous," while China joined calls for the regime of President Bashar al-Assad to respect the former UN chief's plan to end 13 months of violence.

Regime forces killed at least 11 civilians in a new wave of shelling and raids targeting the flashpoint city of Homs and other parts of the country, said the Syrian Observatory for Human Rights.

Activist group the Local Coordination Committees said explosions and heavy gunfire were heard in Damascus and southern Daraa province, cradle of the uprising against Assad's iron-fisted rule.

Shells and rockets were fired into Khaldiyeh, a stronghold for regime opponents who have taken up arms, as well as other parts of Homs, the Observatory and LCC reported.

Shelling and raids also targeted towns and villages in the northern province of Aleppo, the central province of Hama and the coastal province of Latakia, according to the Observatory and activists.

At least seven civilians were killed in Homs on Tuesday, the deadline for Assad to withdraw his troops and weapons from population centres under the peace plan it agreed with Annan.

On the northern border, shots fired by Syrian forces struck a Syrian refugee camp inside Turkey, Turkish media reported, two days after shooting from Syria wounded four Syrians and two Turks on Turkish soil.

Annan, speaking in Tehran, said however the situation should be "much improved" by Thursday's ceasefire deadline if both sides respect his six-point peace plan.

"If everyone respects, I think by six o'clock (0300 GMT) on Thursday the 12th... we a should see a much improved situation on the ground," said the UN-Arab League envoy.

Damascus had given "further clarifications" over how it would implement its side of the plan, he said.

"What they mean and want is an assurance that the other forces, the opposition forces, would also stop the fighting so that we could see cessation of all the violence," he said.

Annan said his team has "had positive answers from them" and that "governments with influence" had also been approached to ensure all sides respect the ceasefire.

He rejected the option of arming the uprising, as proposed by Saudi Arabia and Qatar, saying: "I've always said the militarisation of the conflict will be disastrous."

Annan arrived in Tehran from Turkey. Iran is Syria's principal ally in the Middle East and has given political and material support to Assad's regime.

Turkey, a one-time ally of the Damascus regime, has called for Assad to step down to resolve the bloodshed that has killed more than 9,000 people since it started a year ago, according to UN estimates.

Fifty-two people, including 28 civilians, were killed on Tuesday alone, the Observatory said, bringing the toll since the weekend to at least 337.

Damascus had said Tuesday it was abiding by the plan to withdraw its forces, but Annan accused it of pulling troops from some areas and moving them elsewhere.

"I again appeal to the Syrian government and the Syrian parties to cease violence in accordance (with) the plan," Annan said Tuesday. "I believe there should be no preconditions for stopping violence."

Annan insisted his peace plan was not dead, however.

"We still have time between now and the 12th to stop the violence and I appeal to all concerned, the government in the first place, and the opposition forces," he said.

Beijing, meanwhile, stepped in urging its ally in Damascus, and the opposition, to abide by the peace plan.

"China once again calls on the Syrian government to respond to the six-point proposal," foreign ministry spokesman Liu Weimin said, referring to Annan's plan.

"At the same time, the Syrian opposition should also immediately stop fire and take practical measures to implement" the plan, he told reporters.

Russia, another ally of Syria, also pressured Damascus to implement the Annan plan, with Foreign Minister Sergei Lavrov saying "their action could have been more active (and) more decisive."

Russia and China vetoed two resolutions on Syria because they hinted at sanctions. They have however signed up to less weighty statements which backed Annan's plan and his deadlines to Assad.

Despite the faltering peace plan, the UN Security Council has called on Assad to adhere to Thursday's deadline.

Susan Rice, US ambassador to the United Nations, warned the Security Council faces a looming "moment of truth" on Syria.

The "logical next step" would be to increase pressure through "collective action," she said.

France's Foreign Minister Alain Juppe said he wants "new measures" by the Council after Thursday, and Britain's Foreign Secretary William Hague said it should refer Assad to the International Criminal Court.

At the same time, the Free Syrian Army, a ragtag rebel force of mainly army defectors, warned it would resume attacks if the regime offensive does not stop.

"If (the regime) does not stop shelling and not withdraw tanks, we will intensify our military operations and launch attacks," FSA spokesman Colonel Kassem Saadeddine told AFP.

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