US Defense Secretary Jim Mattis testifies before a Senate Armed Services Committee hearing on Capitol Hill in Washington, US, on April 26, 2018. Photo: REUTERS/Aaron P. Bernstein.

US Defense Secretary Jim Mattis said on Thursday that he expected a “re-energized” effort against Islamic State militants in eastern Syria in the coming days.

“You’ll see a re-energized effort against the middle Euphrates River Valley in the days ahead and against the rest of the geographic caliphate,” Mattis told a Senate Armed Services Committee hearing, referring to territory held by the group.

US airstrikes, troops and US-backed Syrian militias have dealt heavy blows to Islamic State in Syria but the group still holds some areas and is widely expected to revert to guerrilla tactics if the last remnants of its once self-styled “caliphate” are captured.

US officials have said that in recent days they have seen fighters from the US-backed Syrian Democratic Forces (SDF), an alliance of militias in northern and eastern Syria dominated by the Kurdish YPG, returning to the middle Euphrates River Valley to fight against Islamic State.

A Turkish offensive in Afrin against the Kurdish YPG militia, which Ankara regards as an extension of a Kurdish insurgency at home, led to an “operational pause” in the fight against Islamic State in eastern Syria in March.

Mattis added that he was also expecting increased operations against Islamic State militants on the Iraqi side of the border and France had reinforced the fight against the militant group in Syria with special forces in the past two weeks.

US President Donald Trump on Tuesday said he wants to withdraw American troops from Syria “relatively soon” but not before their mission is completed, adding that negotiations over the crisis there should be part of a larger deal regarding Iran.