GENEVA (REUTERS) - The top UN human rights official urged global powers on Wednesday not to supply Syria with weapons and to press both sides in its civil war to find a political solution to prevent more massacres and threats to regional stability.

"If the current situation persists, or deteriorates further, increased inter-communal massacres are a certainty, rather than a risk," said Navi Pillay, addressing an urgent debate at the U.N. Human Rights Council on Wednesday.

"The message from all of us should be the same: we will not support this conflict with arms, ammunition, politics or religion," she told the 47-member Geneva forum.

Pillay spoke after the European Union decided to let an EU arms embargo on Syria expire and Russia said it would deliver an advanced S-300 air defence system to the Damascus government despite US, French and Israeli objections.

France and Britain, the EU's strongest military powers and most ardent advocates of scrapping the embargo, said they had not yet decided to arm Syrian rebels, but wanted to put Assad - who enjoys Russian support - under pressure to negotiate.

The Council addressed the intensified fighting in the 26-month-old Syrian conflict, especially an onslaught by government forces on the rebel-held Syrian border town of Qusair, at the request of Qatar, Turkey and the United States.

"The assault on Qusair is the latest in the regime's attempts to use sectarian-driven war to divide the Syrian people," US Ambassador Eileen Chamberlain Donahoe told the gatherings.

"There is no room for Assad or members of his government who have committed atrocities."

Russian Foreign Minister Sergei Lavrov criticised a U.S.-backed draft resolution before the Council condemning the Syrian government, saying it was "odious" and would undermine efforts to convene an international peace conference.

Syrian Ambassador Faysal Khabbaz Hamoui condemned the Council session as well as Qatar and Turkey, accusing them of being "major parties in the bloodshed in Syria" by helping to"recruit jihadist extremists" from more than 40 countries.

The draft resolution was "biased and politically motivated"he said, adding:" It is far from the truth."