WASHINGTON — Despite the Islamic State group’s territorial losses, it is not surrendering and will morph into an insurgency marked by assassinations, improvised bomb attacks and ambushes, warned America’s top general in the Middle East to Congress on Thursday.

“Reduction of the physical caliphate is a monumental military accomplishment, but the fight against ISIS and violent extremism is far from over,” U.S. Central Command chief Gen. Joseph Votel told the House Armed Services Committee.

ISIS fighters “remain unrepentant, unbroken and radicalized,” he said, noting that they are shrewdly safeguarding their families and capabilities by melting into remote areas and camps for displaced people, “waiting for the right time to resurge.”

“We will need to maintain a vigilant offensive against this now widely dispersed and disaggregated organization that includes leaders, fighters, facilitators and of course their toxic ideology,” Votel said, calling it “a serious generational problem, if not handled correctly.”

His assessment contradicts President Donald Trump, who said in recent weeks the group was on the brink of eradication and had lost 100 percent of the territory it once controlled in Syria. (Votel said ISIS maintains a 1-mile sliver of the 34,000 square miles it once held.)

In December, Trump announced that he was pulling all American troops out of Syria, but partially reversed course and agreed to keep a residual force of perhaps a few hundred troops as part of an international effort to stabilize northeastern Syria.

U.S. and other western countries are backing a Syrian group known as the Syrian Democratic Forces, or SDF, comprised of local Kurdish and Arab fighters who have battled ISIS for more than three years.

On Thursday, Votel predicted that in the next phase, Islamic State fighters might employ “low-level” tactics, but he expressed confidence in the small U.S. force, backed by allies and the ability to mass forces from elsewhere in the region. “We’re going to have to keep pressure on this,” he said.

Democratic lawmakers criticized the administration for not consulting allies and military leaders prior to announcing in December, on Twitter, that ISIS was defeated and that U.S. forces would leave Syria.

In an exchange with House Armed Services Committee Chairman Adam Smith, D-Wash., Votel offered assurances the withdrawal would be driven by the defeat of ISIS, and not an arbitrary timeline.

“There is not pressure on me to meet a specific date at this particular time,” Votel said, punting further details to a closed session.

Smith questioned whether military necessity was actually driving the administration’s decision-making.

“What seems to be driving the withdrawal is the president’s split-second decision to send out a tweet that we’re going to get out of Syria,” Smith said. “The public perception, the international perception is that prior to that tweet, it wasn’t planned.”

The panel’s ranking member, Rep. Mac Thornberry, R-Texas, said it was important to maintain pressure on ISIS, but he was optimistic both that allies would join America’s residual force in Syria and that such a force would be sufficient for the next phase.

“I don’t know it’s enough [of a troop presence]; time will tell,” Thornberry said, “but what I’m incredibly grateful for is we will have a continuing presence to do it, rather than completely leave, which was my big fear.”

The Associated Press contributed to this report.

Joe Gould was the senior Pentagon reporter for Defense News, covering the intersection of national security policy, politics and the defense industry. He had previously served as Congress reporter.

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