WASHINGTON: U.S. forces airdropped small arms ammunition and other supplies to Syrian Arab rebels, barely two weeks after Russia raised the stakes in the long-running civil war by intervening on the side of President Bashar al-Assad. One military official said the drop, by Air Force C-17 cargo planes in northern Syria on Sunday, was part of a revamped U.S. strategy announced last week to help rebels in Syria battling Islamic State militants.

Last week, Washington shelved a programme to train and equip "moderate" rebels opposed to Assad who would join the fight against Islamic State.

The only group on the ground to have success against Islamic State while cooperating with the U.S.-led coalition is a Kurdish militia, the YPG, which has carved out an autonomous zone in northern Syria and advanced deep into Islamic State's stronghold Raqqa province.

On Monday, the YPG announced a new alliance with small groups of Arab fighters, which could help deflect criticism that it fights only on behalf of Kurds. Washington has indicated it could direct funding and weapons to Arab commanders on the ground who cooperate with the YPG.

Amnesty International, in a new report, accused the YPG of committing war crimes by driving out thousands of non-Kurdish civilians and demolishing their homes in Kurdish-controlled areas. A YPG spokesman called it "a false allegation."

The U.S. military confirmed dropping supplies to opposition fighters vetted by the United States but would say no more about the groups that received the supplies or the type of equipment in the airdrop.

Syrian Arab rebels said they had been told by Washington that new weapons were on their way to help them launch a joint offensive with their Kurdish allies on the city of Raqqa, the de facto Islamic State capital.

The Russian intervention in the four-year Syrian war has caught U.S. President Barack Obama's administration off guard. Washington has been trying to defeat Islamic State while still calling for Assad's downfall. Russian President Vladimir Putin was rebuffed in his bid to gain support for his country's bombing campaign, with Saudi sources saying they had warned the Kremlin leader of dangerous consequences and Europe issuing its strongest criticism yet.