An attack to drive ISIS from its Syrian base of operations in Raqqa is expected within weeks, the head of one of the militia expected to take part in the US-backed operation was quoted as saying on Tuesday.

Sipan Hamo, head of the Syrian Kurdish YPG militia, told the London-based al-Hayat newspaper that contacts were underway with the US-led coalition to set "zero hour" for the operation.

The YPG is part of a military alliance including Syrian Arab groups that were airdropped small arms and ammunition by US forces in northeastern Syria on Sunday. The airdrop is part of a revamped US strategy announced last week to help rebels in Syria battling ISIS.