Syrian forces on Monday bombarded the city of Rastan for a second day running, monitors said, as ex-UN chief Kofi Annan and other world envoys prepared a diplomatic drive to end the year-long bloodshed.
UN humanitarian chief Valerie Amos meanwhile said Damascus had finally approved a visit, to take place from Wednesday to Friday, after protests over President Bashar al-Assad's refusal to let her in.
And the Red Cross 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 central Syria.
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 Syrian Observatory for Human Rights said.
It said seven civilians, including four children, were killed on Sunday in shelling of the city, which like Homs straddles the main highway linking Damascus to the north.
"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.
Security forces also launched an offensive in Yabrud in the Damascus region, the Britain-based Observatory said, adding that at least six people, including two teenagers, were killed nationwide on Monday.
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.
"To be clear: this will require the United States to suppress enemy air defences in at least part of the country," McCain said in remarks prepared for delivery on the floor of the Senate.
In Cairo, Arab League chief Nabil al-Arabi told reporters that Annan, who has been named special envoy to Syria for the United Nations and League, will travel to Damascus on Saturday.
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 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.
The General Assembly resolution demands that Damascus "cease all violence and protect its population," free everyone detained in connection with the unrest, pull troops from urban areas and guarantee freedom of demonstration.
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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.
"We're hoping for some fresh attention to the tragedy in Syria now that the elections are past," State Department spokeswoman Victoria Nuland said.
While French Foreign Minister Alain Juppe on Monday urged newly elected Russian president Vladimir Putin to review Moscow's policy of shielding Damascus, the EU urged Moscow to recognise the need for a new Syrian leadership.
"We need to see Russia participate in helping us to achieve that, and to recognize that there needs to be a new leadership in Syria," European Union foreign policy chief Catherine Ashton told reporters in Prague.
In Beijing, meanwhile, foreign ministry spokesman Lu Weimin said China's former ambassador to Damascus, Li Huaxin, would travel to Syria on Wednesday for meetings with the government and other parties.
UN aid chief Amos said 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."
News of the visit came as the International Committee of the Red Cross said it had still not been granted permission to enter Baba Amr, the rebellious district of Homs overrun by regime forces.
"Negotiations are still ongoing," ICRC spokesman Saleh Dabbakeh told AFP, amid mounting international outrage against Assad's regime for its crackdown which the UN says has cost at least 7,500 lives since last March.
Rebel fighters fled Baba Amr last Thursday in the face of a ground assault by regime forces following a month-long shelling blitz which the US-based Human Rights Watch said had killed some 700 people.
Washington, meanwhile, slapped economic sanctions on Syrian state broadcasters, saying they were aiding the regime in its campaign to put down a popular uprising.
The United States has already imposed general sanctions on the entire Syrian government and its agencies, freezing any US assets they might have and forbidding US entities and citizens from any transactions with them.
Canada also imposed new sanctions targeting Syria's central bank and seven ministers, shuttered its embassy in Damascus and a consulate and Ottawa's diplomats left the country, the foreign ministry said.