The Islamic State (IS) jihadist group was "resurging" in Syria while the United States withdrew troops, and has solidified its insurgent capabilities in neighboring Iraq, a Defense Department watchdog said Tuesday.

The jihadists - who suffered major territorial losses at the hands of Iraqi and Syrian forces backed by a US-led international air campaign - are exploiting weaknesses in local forces to make gains, the report from the Office of Inspector General said.

"Despite losing its territorial 'caliphate,' the Islamic State in Iraq and Syria (ISIS) solidified its insurgent capabilities in Iraq and was resurging in Syria this quarter," according to the report, which uses an alternate name for the jihadist group.

IS has been able to "regroup and sustain operations" in the two countries partially because local forces "remain unable to sustain long-term operations, conduct multiple operations simultaneously, or hold territory that they have cleared," the report said.

The group's Syria resurgence came as Washington "completed a partial withdrawal" from the country, a move made despite commanders saying that the local US-backed Syrian Democratic Forces (SDF) "needed more training and equipping for counterinsurgency operations."