The United States is likely to send weaponry like rocket-propelled grenades and mortars to Syria's rebels after President Barack Obama approved arming the insurgents, sources said on Friday.

A source in the Middle East who is familiar with US dealings with the rebels told Reuters that weapon supplies would include automatic weapons, light mortars and rocket-propelled grenades, known as RPGs.

Accusing Syrian President Bashar Assad's forces of using chemical weapons, the White House said on Thursday the United States would supply direct military assistance to the rebels. A US official said that meant sending them weapons supplies for the first time.

Two European security sources said the United States would increase the caliber of the arms and ammunition being supplied to the rebels by regional powers including Saudi Arabia and Qatar, as well as supply some heavier weapons, including RPGs.

More RPGs would give the rebels, who have lost ground to government forces and Lebanese Hezbollah militants in recent weeks, greater ability to fight government armored vehicles and even tanks.

But a US official who has been briefed on the new policy said he did not expect the new US aid to seriously affect the course of events in Syria.

All three sources said there were no plans to send shoulder-held anti-aircraft missiles, known as MANPADS, to the mostly Sunni rebels fighting Assad and his Lebanese Hezbollah Shi'ite allies.

The first military supplies - to be sent to groups under rebel commander Salim Idriss that are vetted by Washington and its allies - could take a minimum of two to three weeks to be delivered.

Deputy national security adviser Ben Rhodes refused to say on Friday how Washington would arm the rebels.

"I'm not going to get into kind of a detailed description of different types of assistance," he told a briefing in the White House.

Aid to the rebels will most likely go through Turkey, where the United States is involved in a secret base that Turkey set up with Saudi Arabia and Qatar to direct military and communications aid to Syria's armed opposition. US aid could also go through Jordan where several thousand US troops are on a joint exercise. A further 200 soldiers from the US Army's First Division are also there.

The commander of Syria's main rebel fighting force urged Western allies on Friday to supply anti-aircraft and anti-tank missiles and to create a no-fly zone, saying if properly armed he could defeat President Bashar Assad's army within six months.

Speaking a day after Washington said it would arm the rebels, Free Syrian Army (FSA) commander Salim Idriss told Reuters his forces urgently needed heavier weapons in the northern city of Aleppo, where Assad's government has said its troops are preparing a massive assault.

"The regime, supported by Hezbollah, is trying to recapture the city and suburbs and we are now in most need. We hope that our friends will not leave us alone," Idriss said after meeting with U.S. and European officials in Turkey.

"If we have done the training ... and have enough weapons and ammunition I think it will be a matter of time, about six months, maybe less, maybe more, to collapse the regime."

Western diplomats said on Friday the United States was considering a no-fly zone in Syria, potentially its first direct intervention into the two-year-old civil war, after the White House said Syria had crossed a "red line" by using nerve gas.

"The no-fly zone is very important because the air force of the regime is very effective," Idriss said.

"They use modern jets to bombard the cities and towns, and civilians and fighters, and we don't have any effective kind of weapons for air defense."

US President Barack Obama has authorized sending US weapons to Syrian rebels for the first time, a US official said on Thursday, after months of deliberation.

The decision came as Assad's forces and their Lebanese Hezbollah allies turned their guns on the north, fighting near Aleppo and bombarding the central city of Homs after seizing the initiative by winning the open backing of Hezbollah last month and capturing the strategic town of Qusair last week.

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