Billowing black smoke was seen pouring into the sky above the Syrian town of Homs on Wednesday after government forces reportedly bombed an oil line.

Live web broadcasts from Homs showed the giant dark cloud hanging over the city.

It was reported that the blast was the result of an attack on an oil line. It was not possible to immediately verify those reports, or to say whether the attack was deliberate.

The dramatic scenes came as Syrian state media reported that it would now hold a "referendum" for a new constitution later in February.

"President Assad has issued a decree to hold a referendum on the 26th of this month for a draft constitution for the Syrian Arab Republic," Sana news said.

Activists reported "the sky raining shells" after more than a week of violence.

Several '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."

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.