Pressure is mounting on President Bashar al-Assad with growing foreign condemnation of his repression of the Syrian uprising and attacks by armed rebels that his forces appear unable to stamp out.

European and Arab diplomats told Reuters the top United Nations human rights forum would announce on Wednesday a special Friday session that was expected to condemn Syria for crimes against humanity.

The move is partly designed to put pressure on China and Russia to take a stronger stand against Assad's government.

The 47-member U.N. Human Rights Council's third session on Syria in eight months is being convened days after a U.N. commission of inquiry said Syrian government forces had committed murder, torture and rape in their crackdown on pro-democracy protests.

"This is very much being led by the Arab group. Some Arab ambassadors are at least as concerned as the European Union and United States and possibly more," Britain's ambassador Peter Gooderham told Reuters.

"There is no question that the resolution will be very hard-hitting at the Council's session on Friday," he said. "It is all intended to build up the maximum pressure that the Human Rights Council can apply."

REBEL AMBUSH

Syrian rebel forces ambushed an army vehicle in northern Syria on Tuesday, killing three soldiers and capturing two others, human rights observers said.

"The security forces vehicle was targeted while driving in the city of Saraqeb in Idlib province by a group of suspected army defectors," the Syrian Observatory for Human Rights said in a statement.

Government forces later killed a civilian and wounded three others in raids in Saraqeb, it said. In a district of Homs city, an 8-year-old girl was shot dead at a checkpoint.

Two civilians died of their wounds in the area of Rinkous outside Damascus on Tuesday and a 33-year-old man was killed by sniper fire as he tried to escape arrest, the Observatory said.

It said families in the area had been unable to bury seven people killed there since Sunday.

The United Nations report on Monday accused Syria of crimes against humanity in the 8-month-old repression of a revolt in which the U.N. says at least 3,500 have been killed.

Syria's northern neighbour Turkey said on Tuesday it feared there could be an exodus of Syrians if the violence got worse, and that border states might have to create a buffer zone. Russia in contrast warned major powers against interference. Turkey said it must prepare for 'any scenario'.

"If tens, hundreds of thousands of people start advancing towards the Iraq, Lebanon, Turkey borders, not only Turkey but the international community may be required to take some steps such as a buffer zone," Turkish Foreign Minister Ahmet Davutoglu said in a television interview.

Turkey has an 800-km (500-mile) border with Syria. It has said it will selectively impose sanctions announced by the Arab League to avoid harming the Syrian people.

France has raised the idea of a secured humanitarian corridor, a step which would appear to imply some use of armed forces for security and logistics, if camps were set up on Syria's border to accommodate masses of refugees.

Russian Foreign Minister Sergei Lavrov said other states should 'stop issuing ultimatums' to Assad and drop talk of an arms embargo which he said would favour the rebels.

"We know how that worked in Libya when the arms embargo only applied to the Libyan army, the opposition received weapons, and countries like France and Qatar publicly spoke about it without shame," Lavrov said.

"For the most part, armed groups are provoking the authorities. To expect the authorities to close their eyes to this is not right," said Moscow's veteran top diplomat.

Syria is a major weapons client of Moscow as well as a long-time strategic ally dating back to Soviet times. It also has close ties with Iran.