A fighter from the Free Syrian Army's Al Rahman legion carries a weapon as he walks towards his position on the front line against the forces of Syria's President Bashar al-Assad in Jobar, a suburb of Damascus, Syria, in this July 27, 2015 file photo. Only four or five US-trained Syrian rebels were still fighting in Syria, a top general told Congress on September 16, 2015, a stark admission of setbacks to a fledgling military program that critics have already pronounced a failure. [Photo/Agencies]

LONDON - The United States will largely abandon its failed efforts to train moderate Syrian rebels fighting Islamic State, and instead provide arms and equipment directly to rebel leaders and their units on the battlefield, the Obama administration said on Friday.

The US announcement marked the effective end to a short-lived $580 million program to train and equip units of fighters at sites outside of Syria, after its disastrous launch this year fanned criticism of President Barack Obama's war strategy.

The Pentagon said it would shift its focus away from training to providing weapons and other equipment to rebel groups whose leaders have passed a US vetting process to ensure they are not linked to militant Islamist groups.

The strategy switch comes as the Obama administration grapples with a dramatic change in the landscape in Syria's four-year-old civil war, brought about by Russia's military intervention in support of President Bashar al-Assad. Moscow's intervention has cast doubt on Obama's strategy there and raised questions about US influence in the region.

Washington's announcement came as Islamic State fighters seized villages close to the northern city of Aleppo from rival insurgents, according to a monitoring group, despite an intensified Russian campaign.

Moscow is mounting air strikes and missile attacks that it says are aimed both at supporting its longtime ally Assad and combating Islamic State. Washington says Russian air strikes in Syria are targeted primarily not at Islamic State but at other rebel groups, including those that have received US support.

Obama has previously questioned the notion that arming rebels would change the course of Syria's war. In an interview with the New York Times in August 2014, he said the idea that arming the moderate Syrian opposition would make a big difference on the battlefield had "always been a fantasy."

By vetting only rebel commanders, the new US policy could raise the risk that American-supplied arms could fall into the hands of individual fighters who are anti-Western.

Christine Wormuth, the Pentagon's No. 3 civilian official, said however that the United States had "pretty high confidence" in the Syrian rebels it would supply, and that the equipment would not include "higher end" arms such as anti-tank rockets and shoulder-fired anti-aircraft rockets.

The Pentagon will provide "basic kinds of equipment" to leaders of the groups, Wormuth, Under Secretary of Defense for Policy, told reporters on a White House conference call.

The Syrian rebel groups that have recently won favour with Washington include Sunni Arabs and Kurds as well as Syrian Christians, US officials have said.

Wormuth defended the Pentagon program launched in May that trained only 60 fighters, falling far short of the original goal of 5,400 and so working out at a cost so far of nearly $10 million per trained fighter.

"I don't think at all this was a case of poor execution," Wormuth said. "It was inherently a very, very complex mission,"

Ben Rhodes, the White House deputy national security adviser, said the new approach showed there had been "deficiencies" in the train-and-equip program that had to be addressed.

When it was launched, the program was seen as a test of Obama's strategy of having local partners combat Islamic State militants and keeping US troops off the front lines. But the program was troubled from the start, with some of the first class of fighters coming under attack from al Qaeda's Syria wing, Nusra Front, in their battlefield debut.

The Pentagon confirmed last month that a group of US-trained Syrian rebels had handed over ammunition and equipment to Nusra Front, purportedly in exchange for safe passage.