Activists in the beleaguered Syrian town of Homs have described "the sky raining shells" as attacks by government forces intensified after more than a week of violence.

As mortars fell on the town for an 11th straight day, 'citizen journalists' working with the protest network Avaaz said that more than 25 shells had exploded in half an hour on Tuesday morning.

"It is as though the sky is raining shells," said one resident known as Basil.

"We can no longer see the sky in Baba Amr. We are covered by a thick blanket of smoke from the bombs."

Video reportedly shot in Homs on Tuesday showed a violent, large-scale explosion erupting in flame and bellowing thick black smoke into the sky.

Other videos showed the sky filled with grey debris and more dark smoke.

Residents said that communications had been cut in Homs and surrounding areas, and that gunfire was heard nearby.

At least 35 people were killed across Syria on Tuesday, Avaaz said, adding that at seven were killed in Homs alone.

There were also clashes reported in the suburbs of Damascus, and a general strike was held in Deir Ezzor where protesters chanted "down with the regime" in defiance of President Bashar al-Assad's forces.

In resuming the shelling, the Syrian regime shrugged off harsh criticism by the United Nations human rights chief Navi Pillay on Monday.

State media said that the government had sent Pillay a letter in "absolute rejection" of her claims.

China, which recently vetoed a security council resolution calling for Assad to leave office, said on Tuesday that it had sent an envoy to Cairo for "frank and useful" talks with the Arab League.

"Given the constantly escalating Syrian situation, the aim of this visit to Cairo was to explain China’s position and policies to the Arab League and Arab countries, and listen to their opinions," the Chinese Foreign Ministry said, according to the AP.

Close to 6,000 people have been killed in Syria since anti-government protests there began in March 2011, the UN said.