Washington: The US has vowed to fight the Islamic State (IS) terror group in Syria "as long as they want to fight", Defence Secretary Jim Mattis said here.

The US forces will not "walk away" to "let IS pop back around", Xinhua news agency quoted Mattis as saying on Monday. "The enemy hasn't declared that they're done with the area yet, so we'll keep fighting as long as they want to fight." IS has been stripped off from its major strongholds in Syria and Iraq over the last few months.

The militants lost their de facto capital of Raqqa last month to the US-backed Syrian Democratic Forces, and this month they lost their major stronghold of Deir al-Zour city to the Russian-backed Syrian government forces.