Syrian troops intensively shelled rebel-held neighbourhoods in the restive central city of Homs Friday and killed at least 13 people, activists said, as Britain and France sought more international support to resist the deadly government crackdown.

Activist groups said tens of thousands of protesters poured into the streets after Friday prayers from Daraa in the south to Aleppo and Idlib in the north and Deir el-Zour in the east to areas around the capital Damascus. The Local Coordination Committees of Syria, an activist coalition, said security forces opened fire on some protests.

In Washington, Secretary of State Hillary Clinton and EU foreign policy chief Catherine Ashton condemned the violence and demanded that Syrian President Bashar al-Assad step down.

In Paris, British Prime Minister David Cameron said that "what is happening in Syria is appalling."

"I'm not satisfied that we are taking all the action we can," he said at a joint newsconference with French President Nicolas Sarkozy. The two leaders spoke a day after the UN General Assembly condemned human rights violations by Assad's authoritarian regime.

"We cannot bring about a Syrian revolution … if the Syrian revolution does not make an effort to rally together and organize so that we can better help them," Sarkozy said. He insisted, however, that "the revolution will not be brought about from outside, it will be brought about from the inside."

Sarkozy, Cameron urge more sanctions

Cameron said Britain and France are working "to see what more we can do" to help the Syrian opposition.

In a joint statement, Cameron and Sarkozy pledged that their countries "will continue to increase their engagement with the Syrian opposition, including encouraging the opposition to work together and to support the vision of an inclusive, prosperous and free Syria. "

They urged the European Union to adopt new sanctions against the regime by Feb. 27 and offer "substantial" aid to Syria if and when Assad leaves.

A frame from an amateur video released Thursday purports to show a Syrian military tank in Daraa, Syria. Syrian troops intensively shelled rebel-held neighbourhoods in the restive central city of Homs on Friday, activists said. (Associated Press/Shaam News Network via APTN)

Cameron also said Britain is sending food rations for 20,000 people and medical supplies for those affected by fighting in Homs and elsewhere in Syria.

The aid will include emergency drinking water and essential household items for refugees forced to leave their homes because of fighting in areas where humanitarian agencies are struggling to work freely amid government restrictions on movement.

Tanks continue shelling Homs

The Britain-based Syrian Observatory for Human Rights said shells slammed into the Homs neighbourhoods of Baba Amr, Bayadah, Khaldiyeh and Inshaat, killing at least 13 people. Syrian troops have been attacking the neighbourhoods since Feb. 4. Amateur videos showed at least one tank shelling Baba Amr from close by.

Homs, a province in central Syria that stretches from the border with Lebanon in the west to the frontiers with Iraq and Jordan in the east, has been one of the key centres of the 11-month-old uprising against Assad. As the uprising has become more militarized in recent months with army defectors battling regime forces almost daily, the rebels have taken control of small parts of the province including neighbourhoods in the city of Homs and the nearby town of Rastan.

Two people were killed by security forces fire in Daraa, and 11 others in various attacks across the country, the group said, raising the overall death toll Friday to 26. Syria has banned many foreign journalists and keeps tight restrictions on the local press, making verification of death toll reports almost impossible.

State-run news agency, SANA, quoted Assad as telling visiting Mauritanian Prime Moulaye Ould Mohamed Laghdaf that political reforms in Syria "will move in parallel with restoring security and stability." It was another clear sign that Assad's regime will continue in its crackdown.