Syrian artillery and warplanes have hit rebel-held areas of Damascus, as Russia has warned against any attempt to establish a no-fly zone over the country.

The pro-opposition Syrian Observatory for Human Rights, a UK-based group, said jets and artillery had attacked Jobar, a battered district where rebels operate on the edge of central Damascus, on Saturday.

It said heavy artillery was also shelling opposition fighters in the provinces of Homs, Aleppo and Deir Ezzor.

Fighting was also reported around dawn on the outskirts of the Palestinian Yarmuk camp in southern Damascus, which also came under regime fire along with southern Al-Hajar al-Aswad.

Outside the capital, loyalist troops fired mortar rounds at several areas including western Moadamiyet al-Sham, southern Sbeineh and the region of Wadi Barada, northwest of Damascus.

Sergei Lavrov, the Russian foreign minister, however, said on Saturday that any attempt to establish a no-fly zone using F-16 fighter jets and Patriot air defence missile systems from Jordan would "violate international law".

"You don't have to be a great expert to understand that this will violate international law," Foreign Minister Sergei Lavrov told a news conference with his Italian counterpart in Moscow.

Western diplomats said on Friday the United States was considering a no-fly zone over Syria, but the White House said later that it would be far harder and costlier to set one up there than it was in Libya in 2011, stressing that the United States had no national interest in pursuing that option.

Lavrov also scoffed at suggestions that Assad's regime would use chemical weapons now in light of its apparent growing advantage against the rebels.

"The regime doesn't have its back to the wall. What would be the sense of the regime using chemical weapons, moreover at such a small quantity?" he said.