WASHINGTON — The U.S.-led military coalition in Iraq has killed more than 8,500 Islamic State fighters since its bombing campaign began in August and has gained the upper hand, the top general overseeing the coalition said Tuesday.

Army Gen. Lloyd Austin, the commander of U.S. Central Command, said the Islamic State, which has controlled key parts of northern and western Iraq since last summer, is no longer capable of seizing and holding new territory.

"He has assumed a defensive crouch in Iraq," Austin told the House Armed Services Committee.

In late January, after other American officials were quoted in news reports as estimating that 6,000 Islamic State fighters had been killed, then-Defense Secretary Chuck Hagel, who fought in the Vietnam war as an enlisted soldier, said it was clear that thousands had been killed but he refused to endorse any specific body count. "I was in a war where there were a lot of body counts every day, and we lost that war," he said.

Austin said that in addition to killing at least 8,500 Islamic State fighters, the U.S.-led bombing has destroyed "hundreds" of the group's vehicles, tanks and heavy weapons. The bombing also has degraded the group's ability to generate revenue by striking oil refineries and crude oil collection sites, particularly in neighboring Syria.

"The fact is that he can no longer do what he did at the outset, which is to seize and to hold new territory," Austin said.

Austin made the remarks in his prepared statement at a hearing on President Obama's request for new legal authority to execute the military campaign against the Islamic State. Austin is a former commander of U.S. forces in Iraq.

The general was speaking amid new uncertainty about the direction of the battle against the Islamic State as Iraqi forces, including Iranian-supported Shiite militias, opened a counteroffensive against the Islamic State in the city of Tikrit.

In the weeks ahead, Austin said, it is likely the Islamic State will conduct limited attacks and "orchestrate horrific scenes" to distract and intimidate.

"But make no mistake. ISIL is losing this fight, and I am certain that he will be defeated," Austin said.