Gulf states called on the UN Security Council on Monday to meet urgently to prevent a massacre in Homs, as pro-government forces in Syria try to wrest the city from rebels fighting to topple President Bashar al-Assad.

Assad's army and allied fighters launched an offensive against rebels in control of the central city on Saturday after scoring victories against the opposition in other parts of the country.

Sunni Gulf states led by Saudi Arabia and Qatar have backed the rebels with arms and money in a fight to topple Assad, who is backed by Shi'ite power Iran and Lebanon's Hezbollah group.

"Considering the Syrian regime's insistence on ethnic and sectarian cleansing, as recently happened near Homs, and its use of chemical weapons against the Syrian people, the continued siege of Homs is inhumane and threatens a massacre," said a statement from the GCC.

"The GCC calls on the Security Council to convene on an urgent basis to lift the siege of the city of Homs," added the statement.

The countries said they were particularly worried about the presence of Hezbollah on the government side, fighting "under the banner" of Iran's Islamic Revolutionary Guards Corps.

GCC foreign ministers met EU foreign representative Catherine Ashton on Sunday to discuss the Syrian conflict and later called on the European Union to arm rebels the immediately.

Saudi Arabia and Qatar have been supplying weapons including anti-aircraft systems, Gulf sources have said, but diplomats in the region say the rebels increasingly need improved training and organisational help from Western countries to avoid defeat.-Reuters