Washington: Syrian rebels trained by the United States gave some of their equipment to the al Qaeda-linked Nusra Front in exchange for safe passage, a US military spokesman said on Friday, the latest blow to a troubled US effort to train local partners to fight Islamic State militants.

The rebels surrendered six pick-up trucks and some ammunition, or about one-quarter of their issued equipment, to a suspected Nusra intermediary on 21-22 September in exchange for safe passage, said Colonel Patrick Ryder, a spokesman for US Central Command, in a statement.

‘If accurate, the report of NSF members providing equipment to al Nusra Front is very concerning and a violation of Syria train and equip program guidelines,’ Ryder said, using an acronym for the rebels, called the New Syrian Forces.

US Central Command, which oversees US military operations in the Middle East, was told of the equipment surrender around 1 p.m. on Friday, Ryder said. Earlier on Friday, Ryder had said all weapons and equipment issued to the rebels remained under their control.

The news was the most recent sign of trouble in a fledgling military effort to train fighters to take on the Islamic State militant group in Syria, where a 4-1/2-year civil war has killed about 250,000 people and caused nearly half of Syria's prewar population of 23 million to flee.

A top US general told Congress last week that only a handful of the rebels are still fighting in Syria, though US military officials said this week that dozens more have since joined them.

US officials have told Reuters that a review is underway that could result in scaling back and reenvisioning the programme.