Syria's opposition says the United States must overcome its fear of Islamists among the rebels fighting to overthrow President Bashar al Assad and start arming the resistance movement.

Islamists are among the most effective fighters against the Syrian leadership, some opposition figures said, and Washington needs to know that these conservative Muslims are a far cry from Afghan-style jihadi militants.

Frustration is growing that the US will not supply weapons to the rebels, who have largely used small arms during the 16-month uprising against Mr Assad and a state apparatus dominated by members of his Alawite minority.

Opposition campaigner Fawaz al Tello said: "We have been kissing the hand of the United States and the rest of the world for the 16 months to intervene. Now, after Assad spared no-one in Syria, the US is surprised that al Qaeda may be operating in the country.

"The US has intelligence on the ground and by clever management it can channel weapons to the right people.

"First it has to give a clear signal that it really wants an end of the Alawite-dominated police state in Syria and not just the sidelining of Bashar."

World powers have agreed that a transitional Syrian government should be set up to end the conflict, in which more than 10,000 people have died.

However, they remain at odds over what part Mr Assad might play.

Officials have made clear that Washington opposes arming the rebels as they lack a unified command and due to concerns that high-tech weapons may fall into the hands of Islamists.

Mohaimen al-Rumaid, a member of the Syrian Rebel Front, said Washington was failing to recognise that the country's Islamist rebels were different from the Taliban fighting Nato-led forces in Afghanistan.

He said: "The Americans have not come to terms with the fact that Islamist elements are among the most effective fighters in Syria and that they are not militant or jihadist in the Afghan sense. Syrians have always been conservative Muslims, but not militant."

US intelligence officials have said weapons funded by sympathisers in Saudi Arabia and Qatar are crossing the Lebanese border to the rebels. These are mostly small arms such as AK-47 automatic weapons, plus some anti-tank guided munitions and rocket-propelled grenades.

Sameh al-Hamwi, a prominent activist based on Syria's border with Turkey, said some rebel groups were adopting Islamist slogans and making jihadist-style videos, but that these were to please their financiers in the Gulf. He denied that political Islam was deeply rooted in the resistance.

US officials said Washington was increasing non-lethal aid to the Syrian opposition.

One official said: "We strongly support efforts to remove Bashar Assad from power. He's a menace to the Syrian people, who have suffered terrible brutality at the hands of his regime. Suggestions that we are not encouraging resistance to the Syrian regime are simply incorrect."

Opposition figures who have met US diplomats to discuss supplies said Washington was looking at ways of providing internet and mobile phone access from neighbouring countries. This would allow rebels to bypass the Syrian networks, which are under government surveillance.

Meanwhile, Turkey's armed forces command said it scrambled a total of six F-16 fighter jets in three separate incidents responding to Syrian military helicopters approaching the border on Saturday, but there was no violation of Turkish airspace.