The United States and the United Kingdom have suspended all non-lethal assistance into northern Syria after Islamic Front forces seized headquarters and warehouses belonging to the opposition's Supreme Military Council (SMC), US and UK embassies spokesmen in Ankara have said.

Fighters from the Islamic Front, a union of six major rebel groups, took control of the Free Syrian Army (FSA) bases at the Bab al-Hawa crossing on Syria's northwestern border with Turkey late on Friday, prompting the US announcement, which was made on the following Wednesday.

Turkey has also shut its side of the border crossing, in Hatay province, due to a reported increase in clashes on the Syrian side, customs sources told Reuters news agency, although there was no immediate confirmation from Turkish officials.

Infighting among rebels

Infighting among Syrian rebels has weakened their efforts to bring down President Bashar al-Assad in the conflict which began as peaceful protests against his rule in March 2011 and has descended into civil war.

It was unclear why the Islamic Front had seized the SMC premises and it was not known if any stock had gone missing.

"As a result of this situation, the United States has suspended all further deliveries of non-lethal assistance into northern Syria," the spokesman said, adding that humanitarian assistance was not impacted because it is distributed through international and non-governmental organisations.

Following the US move, a spokesman for the British embassy in Ankara told AFP news agency: "We have no plans to deliver any equipment while the situation remains unclear."

Britain is "in contact with the SMC (Supreme Military Council)" to investigate the status of British equipment, the spokesman said, referring to the highest military authority in the FSA.

Under title 10 of the US code that defines the role of the US armed forces, non-lethal aid could consist of communications equipment, medical supplies, intelligence assistance and body armour.

FSA representatives could not be reached for comment and the US embassy spokesman said the situation was being investigated "to inventory the status of US equipment and supplies provided to the SMC".