Syrian forces pursued armed rebels on Tuesday in a number of flashpoint areas ahead of a peace mission by UN-Arab League envoyKofi Annan, as a top US senator called for air strikes to protect civilians.

Soldiers in tanks and armoured carriers stormed the town of Herak in the southern province of Daraa, the Syrian Observatory for Human Rights said, adding that it was not possible immediately to determine whether there had been casualties.

And army deserters clashed with troops overnight in villages of Deir Ezzor, eastern Syria, while in Hama province of central Syria, troops backed by tanks circled the town of Tibet al-Imam, said the Britain-based Observatory.

On Monday, Syrian forces bombarded the city of Rastan, a rebel bastion in neighbouring Homs province for a second day running, monitors said.

The Red Cross, meanwhile, negotiated for a fourth day with Syrian authorities to be allowed to deliver aid and evacuate the wounded from the battered Baba Amr rebel district of the city of Homs in the centre of the country.

The Observatory said at least six people, including two teenagers, were killed nationwide on Monday.

Rastan, which activists expect to be the next target of a drive by regime forces to expel rebels who have regrouped from Homs, 20 kilometres (12 miles) away, came under renewed shelling, the Observatory said.

"What's happening in Rastan is exactly what happened in Baba Amr: a siege, artillery fire and rockets," said Hadi Abdullah, an activist in Homs of the Syrian Revolution General Commission.

"Free Syrian Army fighters are still in Rastan. They will not give up because nobody wants another Baba Amr," he said.

The violence in Homs province has sent more than 1,500 Syrian refugees, mainly women and children, fleeing across the border into Lebanon in the past few days, UN and Lebanese officials said on Tuesday.

With diplomatic efforts so far stymied, US Senator John McCain, an influential Republican, called for American air strikes on Syrian forces to protect population centres and create safe havens.

"Time is running out," McCain -- a hawk on both Syria and Iran -- said in remarks on the floor of the Senate.

At the request of the Syrian opposition, McCain said, "the United States should lead an international effort to protect key population centres in Syria, especially in the north, through air strikes on Assad's forces."

On the diplomatic front, former UN chief Kofi Annan is on Wednesday to launch a mission aimed at convincing Syrian President Bashar al-Assad to silence the guns blamed for thousands of deaths since anti-regime protests broke out last March.

Annan is to hold talks with Arab leaders in Cairo before he heads to the Syrian capital on Saturday as joint special envoy for the United Nations and the 22-member Arab League.

The seasoned negotiator himself called it "a tough challenge" last week when he was named to the post.

The immediate aim of his first Damascus talks will be to secure a humanitarian pause in the fighting and access to the protest centres where the United Nations says more than 7,500 people have been killed.

Then, Annan said, he would "work with the Syrians in coming up with a peaceful solution which respects their aspirations and eventually stabilises the country."

He will be accompanied by his deputy, former Palestinian foreign minister Nasser al-Qudwa, a nephew of Yasser Arafat, on their first trip to Syria, where state media has welcomed the mission.

The two envoys are to serve under a mandate set out by a UN General Assembly resolution passed last month and Arab League resolutions on the crisis.

Russian Foreign Minister Sergei Lavrov, meanwhile, said he was to meet his Arab counterparts on Saturday in Cairo where the League has its headquarters to discuss Moscow's ally Syria.

Moscow and Beijing have since October twice wielded their Security Council veto to block UN condemnation of Syria.

Washington said on Monday it hoped that with the Russian presidential elections over, Moscow would now turn its attention to Syria and push for humanitarian relief to civilians.

"We're hoping for some fresh attention to the tragedy in Syria now that the elections are past," State Department spokeswoman Victoria Nuland said.

Adding to the diplomatic flurry, China's former ambassador to Damascus, Li Huaxin, is due in Syria on Wednesday for meetings with the government and other parties.

And UN humanitarian chief Valerie Amos said Damascus has finally approved a visit, to take place from Wednesday to Friday.

The aim of her visit would be "to urge all parties to allow unhindered access for humanitarian relief workers so that they can evacuate the wounded and deliver essential supplies," she said.