The US is to end its efforts to train new Syrian rebel forces and says it will shift to providing equipment and weapons to existing forces.

Its $500m programme was heavily criticised after it emerged that US-trained rebels had handed vehicles and ammunition over to extremists and that four or five of the fighters were in Syria.

The programme had aimed to train and equip 5,400 fighters this year and a further 15,000 in 2016.

A senior administration official said the programme was being put on "pause".

The programme had suffered from "significant challenges", the official said.

The US will no longer vet every individual recruit but just the leaders of the groups they decide to work with, who will face "very vigorous vetting".

Of the initial two groups sent into the country under the previous programme, the first was rounded up by Jabhat al-Nusra, an offshoot of al-Qaeda, in July. The second handed much of its equipment over to the same group in September, reportedly in exchange for safe passage.

Quoting an anonymous US Department of Defense source, the New York Times reported that the US would no longer recruit Syrian rebels to go through its training programmes in Jordan, Qatar, Saudi Arabia or the United Arab Emirates.