Syrian forces backed by helicopters killed dozens of pro-democracy protesters on Friday, witnesses said, in the first reported use of air power to quell unrest in Syria's increasingly bloody three-month-old uprising.

Meanwhile, the United States and its allies are stepping up pressure on President Bashar al-Assad, to step down or allow for a political transition, a State Department spokesman said.

The use of the aircraft came on a day of nationwide rallies against Assad, as unrest showed no sign of abating despite the harsh crackdown by his authoritarian state.

The helicopters opened fire in a northwestern town after security forces on the ground killed five protesters, said the Syrian Observatory for Human Rights.

'At least five helicopters flew over Maarat al-Numaan and began firing their machineguns to disperse the tens of thousands who marched in the protest,' one witness said by telephone.

'People hid in fields, under bridges and in their houses, but the firing continued on the mostly empty streets for hours,' said the witness, who gave his name as Nawaf.

Syria's state television, in contrast, blamed violence in the area on anti-government groups. It made no mention of attack helicopters but said an ambulance helicopter had come under fire over Maarat from 'terrorist armed groups,' injuring crew.

Britain, France, Germany and Portugal have asked the UN Security Council to condemn Assad, though veto-wielding Russia has said it would oppose such a move.

Denouncing the Syrian government's actions, the White House said Friday's 'appalling violence' had led the United States to back the European draft resolution at the United Nations.

'The Syrian government is leading Syria on a dangerous path,' the White House said.

Foreign Minister Walid al-Moualem wrote to the Security Council accusing the opposition of violence and sabotage, Al Arabiya television said. Foreign governments were basing their views on 'inaccurate information,' it said.

State television said earlier well-armed 'terrorist groups' had burned police buildings and killed members of the security forces in Maarat al-Numaan, which lies 55 km south of Syria's second city Aleppo on the highway to Damascus.-Reuters