WASHINGTON - The US military said Monday it had airdropped small arms ammunition for Syrian rebel groups currently fighting the Islamic State (IS) inside Syria.

"Coalition forces conducted an airdrop Sunday in northern Syria to resupply local counter-ISIL ground forces as they conduct operations against ISIL," said Steve Warren, spokesman for the US-led coalition campaign, using another acronym of ISIL for the extremist group.

According to Warren, the airdrop mission was aimed at arming Syrian rebel groups whose leaders were "appropriately vetted by the United States and have been fighting to remove ISIL from northern Syria."

"Due to operational security, we will not have any further details about the groups that received these supplies, their location, or the type of equipment in the airdrop," he added.

Citing US defense officials who spoke on condition of anonymity, the US TV network CNN said the US military delivered 50 metric tons of ammunition to rebel groups in northern Syria by using an air drop of 112 pallets.

The airdrop marked the first step since the Obama administration announced last week its plan to shift from building a Syrian force against the IS to supporting existing rebel groups.

The shift was a recognition of the failure of US President Barack Obama's flagship anti-IS training program which initially sought to recruit 5,400 Syrian rebels each year for three years.

In a congressional hearing held in September, General Lloyd Austin, who oversees the war against the IS, told US lawmakers that only "four or five" US-trained Syrian rebels currently remained in fight in Syria and the US military would not reach its goal of training 5,400 Syrian fighters any time soon.

Later, the Pentagon corrected the number of US-trained Syrian rebels currently fighting the IS in Syria to nine.

Senior officials of the Obama administration had said earlier the initial US strategy to recruit, train and arm moderate Syrian rebels failed partly because most of Syrian rebels were more focused on fighting the Syrian government.

The Pentagon admitted on Sept 25 that US-trained Syrian rebels gave about a quarter of US weapons to an al-Qaeda affiliate in Syria.