The US Department of Defense warned on Monday that the ISIS terrorist group in Syria could regroup if counter-terrorism measures were not put in place in the war-torn country. ISIS militants have lost territory since Trumps surprise announcement in December that he was pulling US forces out, but military officials warn the fighters could regroup within six months to a year after the Americans leave. The ISIS group "remains a potent force of battle-hardened and well-disciplined fighters that could likely resurge in Syria absent continued counter-terrorism pressure," the report from the inspector general said according to The Associated Press. Trumps decision to leave Syria, which he initially said would be rapid but later slowed down, shocked US allies led to the resignations of Defense Secretary Jim Mattis and the top envoy to the anti-ISIS coalition, Brett McGurk. The withdrawal will fulfill Trumps goal of bringing troops home from Syria, but military leaders have pushed back for months, arguing that ISIS remains a threat and could regroup. US policy has been to keep troops in place until the extremists are eradicated. US officials in recent weeks say ISIS has lost 99.5 percent of its territory and is holding on to fewer than 10 square kilometers of turf in Syria. In late November and December that figure had been estimated at between 400 and 600 square kilometers, according to officials briefed on the matter. But several defense officials said Monday that many fighters fled to ungoverned spaces and other pockets in the north and in the west and are likely hiding out until they can regroup. Trump said in a weekend interview that ISIS is "almost knocked out." "Were at 99 percent right now, well be at 100," he said on CBS "Face the Nation." US officials say that ISIS fighters hold only several villages in the Middle Euphrates River Valley that amount to significantly less than 10 square kilometers. But, they say they do not expect that area to be cleared of militants for another several weeks, at best. Officials say that overall, there are about 2,000 ISIS militants in Syria. The Defense Department watchdog report warned that even with the ISIS forces on the run, the group "is still able to coordinate offensives and counter-offensives, as well as operate as a decentralized insurgency,” said the AP. The report, which covers October through December 2018, also includes a classified section that was provided to Congress and includes a more detailed Pentagon assessment on the impact of the troops withdrawal and the status of ISIS militants and other foreign fighters in Syria. According to the report, US Central Command believes that ISIS fighters will continue to conduct "opportunistic attacks" on US troops as they withdraw. And it says, "If Sunni socio-economic, political, and sectarian grievances are not adequately addressed by the national and local governments of Iraq and Syria it is very likely that ISIS will have the opportunity to set conditions for future resurgence and territorial control."