THE US is considering a no-fly zone in Syria, potentially its first direct intervention into the two-year-old civil war, after the White House said the Assad regime had crossed a "red line" by using nerve gas.

Moreover, President Barack Obama's administration has said it would now arm rebels, having obtained proof the Syrian Government used chemical weapons against opposition fighters.

"Washington is considering a no-fly zone to help Assad's opponents," one senior Washington diplomat said. He added that it would be limited, "time-wise and area-wise, possibly near the Jordanian border", but gave no more details.

Mr Assad's main advantage so far has been the ability to use air power to re-supply troops and bomb rebel areas, along with its use of long-range missiles. But western military support for rebels or a no-fly zone would change the balance of power.

Imposing a no-fly zone would require the US to destroy Syria's sophisticated Russian-built air defences, which Washington says it has not ruled out, but a decision is not imminent.

"A no-fly zone would carry with it great and open-ended costs for the US and the international community," said US deputy national security adviser Ben Rhodes. "It's far more complex to undertake the type of effort in Syria than in Libya."

Any such move would also come up against a potential veto from Mr Assad's ally, Russia, in the UN Security Council.

The Kremlin has dismissed US evidence of Mr Assad using nerve gas, which Washington believes has killed 100 to 150 people.

"I will say frankly that what was presented to us by the Americans does not look convincing," Russia's senior foreign policy advisor Yuri Ushakov said. "It would be hard even to call them facts."

Nevertheless, Washington has quietly moved Patriot surface-to-air missiles, war planes and more than 4000 troops into Jordan in the past week, ostensibly as part of an annual exercise.

US officials say Mr Obama will try to persuade Russian premier Vladimir Putin to abandon support for Mr Assad when the two leaders meet at a G8 summit in Northern Ireland next week.

David Cameron has backed Washington's "candid assessment" that Mr Assad has used chemical weapons, and admitted it left international leaders facing difficult questions.

The Prime Minister said: "I welcome this candid assessment by the Americans. It puts back centre-stage the question we have got to address: What are we going to do about the fact that in our world today there is a dictatorial and brutal leader who is using chemical weapons under our noses against his own people?"

Foreign Secretary William Hague said the crisis in Syria demanded a "strong, determined and co-ordinated response from the international community".

Mr Hague said: "We have to be prepared to do more to save lives, to pressure the Assad regime to negotiate seriously, to prevent the growth of extremism and terrorism, and to stop the regime using chemical weapons against its people."

In the latest round of fighting, Assad forces tried overnight into yesterday morning to storm the rebel-held eastern districts of Aleppo, Syria's biggest city and commercial hub, sparking some of the fiercest battles in months.

Activists also reported artillery and air strikes in the rebel-held countryside in the north.