Islamic State fighters are in disarray and struggling to fend off a rapid offensive by Iraqi forces to recapture Mosul and expel the militants from their last major stronghold in the country, a top U.S. military official said.

"They’re lacking purpose motivation and direction,” Army Maj. Gen. Joseph Martin said in a phone interview from Baghdad, referring to the Islamic State. “I’ve never seen them so disorganized.”

The pace of the battle reflects dramatic improvements in Iraq’s military and its ability to coordinate operations with a U.S.-led air campaign, which is pounding the militants at a record pace.

"You're watching ISIS be annihilated," Martin said of the militant group.

Iraq’s military quickly penetrated a set of obstacles, including concrete barriers and roadside bombs, that the militants had established to slow the Iraqi advance, Martin said.

The militants are struggling to organize military operations to slow the Iraqi advance. "They're taking longer to react to initiatives on the battlefield," he said of the militants.

Iraq's military has been battling ISIS since the militants invaded Iraq from Syria in 2014 with barely any resistance. U.S. training and guidance have resulted in a much stronger fighting force that has retaken major cities such as Ramadi and Fallujah. Mosul is the country's second-largest city, after the Iraqi capital Baghdad.

The militants' current disarray may be the result of repeated airstrikes that have wiped out battlefield leaders, making it difficult for them to organize an effective response to Iraq's improved military.

In addition, ISIS militants are badly outnumbered by Iraqi forces. More than 50,000 Iraqi security forces have been involved in the Mosul offensive. They faced an estimated 3,000 to 5,000 militants defending the city when Iraqi forces initially launched the offensive in October.

In an earlier battle to recapture east Mosul, Iraqi troops were slowed by car bombs, which were deployed by militants to attack the flanks of advancing Iraqi forces.

This week, the militants attempted a counterattack west of the city but were quickly rebuffed by Iraqi ground forces and coalition aircraft. “The counterattack was destroyed,” Martin said. “Their morale has to be pretty low,” he said of the militants.

Iraqi forces have also “disrupted” a key militant capability: the use of drones to conduct reconnaissance of Iraqi positions, Martin said. He declined to detail how that was accomplished, citing security concerns.

Iraq's military, which has captured key government buildings, estimated that it has retaken about 30% of west Mosul since the assault began Feb. 19, and it is beginning to clear dense neighborhoods of radical fighters.

By January, Iraq’s government announced that the east part of the city had been liberated. Several weeks later, forces launched an offensive to take the west part of the city, which is divided by the Tigris River.

U.S. advisers are assisting Iraqi units but primarily from a distance to avoid direct combat with ISIS fighters.

In recent days, Iraqi forces completed the isolation of west Mosul by securing control over the last road out of the city. The successful operation is designed to prevent militants from bringing in supplies and reinforcements from Tal Afar, an ISIS-held town west of Mosul.

“ISIS is trapped,” Brett McGurk, a U.S. envoy to the anti-ISIS coalition, told reporters during a visit to Iraq this week.

Still, U.S. officials expect tough resistance as Iraqi forces clear the neighborhoods in the old city, where militants are holed up in buildings and alleyways.

“I do not want to understate the very difficult fight that lies ahead,” McGurk said.