The Islamic State (ISIS) jihadist group used the withdrawal of US troops from Syria and the Turkish incursion to regroup and strengthen its abilities to plan terror attacks abroad, a new internal report from the Defense Department's watchdog quoted by NBC News on Tuesday says.

The report, from the Pentagon's inspector general, also says ISIS will likely rebound from the elimination of its leader, Abu Bakr al-Baghdadi.

The report identified the October 9 Turkish move into Syria — three days after a phone call between President Donald Trump and Turkish President Recep Tayyip Erdogan — as the tipping point. The invasion affected the US mission against ISIS, the US relationship with its chief military ally in the fight against ISIS — the Syrian Democratic Forces — and control of territory in northeastern Syria.

The departure of US forces from areas of northeastern Syria, which followed the Turkish incursion, then created a void which Syrian regime forces and Russian forces filled, and gave ISIS more freedom to rebuild, according to the report.

"ISIS exploited the Turkish incursion and subsequent drawdown of U.S. troops to reconstitute capabilities and resources within Syria and strengthen its ability to plan attacks abroad," according to a Defense Intelligence Agency analysis of open sources, the report said.

In addition to Turkish forces, Russian and Syrian regime troops entered northeastern Syria in October. All three forces are unlikely to prioritize fighting ISIS, the report said. The largely Kurdish Syrian Defense Forces have pledged to continue operations against ISIS, but were attacked by Turkish forces. It was "unclear how many of the roughly 100,000 SDF forces were still conducting counter-ISIS operations in northeastern Syria" at the time of the report's publication.

The report also stated that the death of Baghdadi in a late-October raid conducted by US special operations forces in Syria was a "significant blow" to the terror group, but would likely have little effect on its ability to rebuild.

The inspector general report covered a larger time period than its typical quarterly report on US efforts to defeat ISIS in Iraq and Syria, "due to the significant events that took place in Syria in October 2019." Typically, such a report would only have covered the period of July through September. The new report stretches almost an extra month to cover those significant events, noted NBC News.

Trump's abrupt announcement in October that he had ordered a full troop withdrawal drew angry rebukes at home and abroad, with critics saying it could allow a resurgence of ISIS.

Defense Secretary Mark Esper subsequently said that the US is mulling the possibility of maintaining a small ground presence near the oil fields in the area.

General Mark Milley, chairman of the Joint Chiefs of Staff, said last week that US troop levels in northern Syria will probably stabilize around 500.