ISTANBUL (REUTERS) - Britain joined the United States in suspending 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 and the general chaos engulfing the nation.
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.
The rebel Free Syrian Army (FSA) 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 powerful Al-Qaeda-backed Islamist militants who now dominate the war with Assad.
The US and Britain have in the past offered radios, body armour, medical supplies, money and food to rebels fighting Mr Assad, but a US embassy spokesman in Turkey declined to give details of what supplies may have been halted.
Fighters from the Islamic Front, which groups six major rebel brigades and which said last week it had quit the FSA, seized headquarters of the Syrian Military Council (SMC), nominally in charge of the FSA, and weapons warehouses at the Bab al-Hawa crossing on Syria's northwestern border with Turkey.
The Syrian Observatory for Human Rights, a British-based anti-Assad monitoring group, said the Islamic Front had seized dozens of 'Shilka' anti-aircraft weapons and anti-tank rockets from the SMC arms stores in fighting on Friday and Saturday.
"I... want to underline that our support to the opposition remains undiminished," the British embassy spokesman said.
"We have been long-standing and strong supporters of General Idriss and the SMC. That remains the case. It is important that the SMC remains united in the face of attacks from the regime and from extremist groups.
"Otherwise this will be a setback for all Syrians who support a political solution and a democratic, pluralist future for their country."
The Islamic Front's battlefield success in capturing the weapons stores could undermine SMC assurances to the US 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.
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 "meals ready to eat food", 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 said that 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".
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 (UN). The first UN relief airlift to Syria from neighbouring Iraq will deliver food and winter supplies to the mostly Kurdish north-east over the next 10 days.
The 2-3/4 year conflict has killed more than 100,000 people, driven more than two 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 spokesman Meqdad said SMC leader General Salim Idriss was talking to the front's leaders to try to resolve the confrontation.
Asked whether any FSA stock was missing, he 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 Mr Assad in a conflict which began with peaceful protests against his rule in March 2011 and has descended into civil war.
Mr Assad's army, backed by Lebanon's Hezbollah militant group and Iraqi Shi'ite fighters, has made steady gains around Damascus and to the north of the capital, while rebel territory in the north has seen increasing cases of inter-rebel conflict.
Many activists who helped to organise protests against Assad have now fled abroad from rebel-held territory, fearing not Assad's security forces but hard-line Islamists they say are equally intolerant of dissent.
Prominent human rights lawyer Razan Zaitouneh was kidnapped in the rebel town of Douma, activists said on Tuesday. They said it was not clear who had seized the 36-year-old activist who has documented human rights violations in Syria.
The family of two Spanish journalists said on Tuesday they have been held since September by fighters linked to Al-Qaeda.
Fifty-five journalists have been killed and 30 are still missing in Syria, according to the New York-based Committee to Protect Journalists, making the country the most dangerous place in the world for media workers.