WASHINGTON — President Donald Trump was expected to declare near-total triumph over the Islamic State group in Syria in his State of the Union address Tuesday, but U.S. defense officials are increasingly fearful that the militants are simply biding their time until the Americans leave the battlefield as planned.

IS militants have lost territory since Trump’s surprise announcement in December that he was pulling U.S. forces out, but military officials warn the fighters could regroup within six months to a year after the Americans leave.

A Defense Department watchdog report released Monday warned of just such a possibility.

The Islamic State group “remains a potent force of battle-hardened and well-disciplined fighters that ‘could likely resurge in Syria’ absent continued counterterrorism pressure,” the report from the inspector general said.

The top commander of U.S. forces in the Middle East, Gen. Joseph Votel, told a Senate committee on Tuesday that of the 34,000 square miles of territory that IS once held, it now controls less than 20 square miles. That figure includes a large swath of desert around Syria villages IS controls.

“It is important to understand that even though this territory has been reclaimed, the fight against ISIS and violent extremists is not over and our mission has not changed,” Votel, commander of U.S. Central Command, told the Senate Armed Services Committee.

“The coalition’s hard-won battlefield gains can only be secured by maintaining a vigilant offensive against the now largely dispersed and disaggregated ISIS that retains leaders, fighters, facilitators, resources and the profane ideology that fuels their efforts.”

Votel said there are now between 1,000 and 1,500 IS fighters in the small area they still control in the southern part of the Euphrates River Valley near the Iraqi border. The remainder, he said, have “dispersed” and “gone to ground,” suggesting they retain the potential to return.

Trump’s decision to leave Syria, which he initially said would be rapid but later slowed down, shocked U.S. allies led to the resignations of Defense Secretary Jim Mattis and the top envoy to the anti-IS coalition, Brett McGurk.

Votel was asked at Tuesday’s hearing whether he was asked for his advice about a Syria withdrawal before Trump announced his decision.

“I was not consulted,” the general said.

The withdrawal will fulfill Trump’s goal of bringing troops home from Syria, but military leaders have pushed back for months, arguing that IS remains a threat and could regroup. U.S. policy has been to keep troops in place until the extremists are eradicated.

Fears that IS fighters are making strategic maneuvers ahead of a U.S. pullout could also fuel criticism that Trump is telegraphing his military plans — the same thing he accused President Barack Obama of doing in Afghanistan.

U.S. officials in recent weeks say IS has lost 99.5 percent of its territory and is holding on to fewer than 5 square kilometers of turf in Syria — less than 2 square miles — in the villages of the Middle Euphrates River Valley, where the bulk of the fighters are concentrated. 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 the caliphate is “almost knocked out.”