"One US service member died today during an Iraqi Security Force mission in... Iraq, while advising and accompanying the ISF during a planned operation," US Central Command said in a statement.

Iraqi forces periodically launch operations in the country's west and northwest border areas, targeting Islamic State group sleeper cells, with support from a US-led coalition.

Iraq's government in late 2017 declared victory against IS, which seized vast swathes of the country including the key northern city of Mosul in a lightning 2014 offensive.

But the jihadist group has lately "solidified its insurgent capabilities in Iraq" , according to a report this week by the US Defense Department watchdog.

The report also said IS was "resurging" in neighbouring Syria, despite a Kurdish offensive -- backed by US-led coalition air support -- to seize the last shred of the jihadists' self-styled caliphate in Deir Ezzor province earlier this year.