Istanbul: The United States and Britain suspended non-lethal aid to northern Syria after Islamist fighters seized Western-backed rebel weapons warehouses, highlighting fears that supplies could end up in the wrong hands.

The rebel Free Syrian Army fighting President Bashar al-Assad said the US and British moves were rushed and mistaken. "We hope our friends will rethink and wait for a few days when things will be clearer," FSA spokesman Louay Meqdad said.

The suspension underlines a crisis for the FSA leadership, which needs international backing to reinforce its credibility and to stop its fighters joining al Qaeda-backed Islamist militants who now dominate the war with Assad.

Fighters from the Islamic Front, which groups six major rebel brigades and which said last week it had quit the FSA, seized the headquarters of the Syrian Military Council, nominally in charge of the FSA, and weapons warehouses at the Bab al-Hawa crossing on Syria's northwestern border with Turkey.

A US official said FSA leader General Salim Idriss had fled into Turkey during the takeover of the warehouses, which contained trucks, food, medical packs and communication equipment including laptops and radios.

The Syrian Observatory for Human Rights, a British-based anti-Assad monitoring group, said the Islamic Front had seized anti-aircraft and anti-tank weapons from the SMC arms stores in fighting on Saturday.

The Islamic Front's battlefield success in capturing the stores could undermine SMC assurances to the United States that no supplies sent to their fighters would fall into the hands of Islamist brigades.

The US embassy spokesman in Ankara said the situation was being investigated "to inventory the status of US equipment and supplies provided to the SMC".

"As a result of this situation the United States has suspended all further deliveries of non-lethal assistance into northern Syria," the spokesman said. Deliveries into southern Syria, through Jordan, would not be affected.

Five rebel fighters were killed in the clashes at Bab al-Hawa, but it was not clear which side they were on.

American aid, including trucks, ambulances and ready meals, reaches Syria overland through Turkey.

US officials said in the summer that they had developed a system of distribution using SMC operatives that would ensure the aid reached US-allied groups. The United States has been concerned the non-lethal aid should not reach Islamists.

A senior US administration official said the suspension should not be misinterpreted.

"This is absolutely not the beginning of the US washing its hands. We will remain engaged in the humanitarian effort. We will remain engaged in the diplomatic effort," the official said, adding: "This doesn't represent a change in policy in our support for the moderate opposition."

He said the administration was looking for other ways to see how the support can be provided to ensure it does not fall into the hands of "extremists".

The British wanted the situation clarified after the clashes. "We have no plans to deliver any equipment while the situation remains so unclear. We will keep this under close review," a spokesman from the British embassy in Ankara said.

Turkey shut its side of the border crossing in Hatay province, customs sources told Reuters, citing a reported increase in clashes on the Syrian side. There was no immediate confirmation from Turkish officials.

Wednesday's announcement does not affect humanitarian support because this is distributed through aid groups and the United Nations. The first UN relief airlift to Syria from neighbouring Iraq will deliver food and winter supplies to the mostly Kurdish northeast over the next 10 days.

The 2-3/4 year conflict has killed more than 100,000 people, driven more than 2 million abroad as refugees and left many millions more dependent on aid.

Playing down the fighting between the Islamic Front and the FSA brigades as a "misunderstanding", the FSA's Meqdad said Idriss was talking to the front's leaders to try to resolve the confrontation.

Asked whether any FSA stock was missing Meqdad said: "Everything will be clear in the next hours, and we believe the Syrians are good people and we don't believe there was a problem. They are our brothers."

Infighting among Syrian rebels has weakened their efforts to bring down Assad in a conflict that began with peaceful protests against his rule in March 2011 and has descended into civil war.

Reuters