WASHINGTON (AFP) - The US military has seen the Islamic State in Iraq and Syria (ISIS) group "resurge" in some parts of Syria under government control, a US defence official said on Tuesday (April 17).

Colonel Ryan Dillon, a spokesman for the US-led coalition fighting ISIS in Syria and Iraq, said the regime of Syrian President Bashar al-Assad and its ally Russia have not always been able to hold the terrain recaptured from ISIS.

"As we look at ISIS in areas where we are not operating, where we are not supporting our partners on the ground, there has been ISIS elements who have been able to come back and take territory (including in) some of the neighbourhoods in southern Damascus," Dillon said, using an alternative acronym for the extremist group.

"We've seen ISIS start to resurge in areas west of the Euphrates River," he added.

The US military is closely watching ISIS in Syria and Iraq, where the militants have lost 98 per cent of the land they once held, according to the Pentagon.

Progress, however, has ground to a halt in recent weeks in areas where the US-led coalition is fighting ISIS through the Syrian Democratic Forces, a local proxy group, due to Turkish military action in the north.

Ankara in January launched a bloody operation around Afrin to push Kurdish fighters out of the city.

Many of the Kurdish fighters that had been engaged in anti-ISIS operations have quit that fight to support comrades in Afrin.

Dillon said there had been no significant gains against ISIS since the Kurdish members of the SDF had left.