Tom Vanden Brook

USA TODAY

WASHINGTON — A U.S. air strike near Mosul on Thursday killed 40 Islamic State militants near a terrorist training camp, according to a senior Defense Department official.

A B-1 bomber was involved in the attack, said the official who was not authorized to discuss the details of the operation publicly. Mosul, Iraq's second-largest city, fell to militants in June.

The air strike is part of the widened campaign against the militant group that President Obama ordered last week. The first round of the new attacks began Monday, and Thursday's was the most deadly against the group, which is also known as ISIL or ISIS. It's apparently the second time a B-1 bomber has been involved in the 176 air strikes that began in August to protect U.S. personnel in Iraq, critical infrastructure such as dams and to prevent ISIL fighters from killing religious refugees.

The attack destroyed "a large ISIL ground unit," according to U.S. Central Command, which oversees military operations in the Middle East. The strike also blew up an armed vehicle and two "ISIL-occupied buildings."

The strike won't defeat the Islamic State, said Michael O'Hanlon, a military analyst with the Brookings Institution. But it can keep them from advancing.

"We won't decimate ISIS with air strikes that kill 40," O'Hanlon said. "But if we can convince them not to go on the offensive, we'll buy time for the rest of the strategy to unfold."

On Capitol Hill this week, top military officials, including Defense Secretary Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, stressed that the U.S. effort in the war will be limited to air power, and advising, training and equipping ground forces from other nations.

Hagel told the House Armed Service Committee on Thursday that U.S. ground troops will not be involved in the fighting. His remarks seemed to slam the door on the possibility of U.S. forces involved in the fighting raised on Tuesday by Dempsey, who said he would recommend spotters for airstrikes or sending advisers close to the fighting if needed.

Dempsey referred specifically to retaking Mosul. Rooting out militants from urban terrain will be a complex operation that could be aided by U.S. advisers, he said.

And a senior Air Force commander told USA TODAY that air strikes will have to be restricted to the outskirts of Mosul unless U.S. commandos are on the ground to direct bombing runs. He was not authorized to speak publicly about the operations.

Air strikes, the commander said, could prevent movement to and from the city, and effectively isolate it, allowing Iraqi forces to retake it. U.S. pilots and planners could coordinate with Iraqi security forces to ensure that their military vehicles were not confused with ISIL trucks.

It is also possible to determine, through extensive use of spy planes, the sorts of buildings that ISIL militants use for headquarters. So-called pattern of life analysis, which tracks the comings and goings of people, can help determine the use of a building.

But while air strikes with 500-pound bombs could be used inside an urban area such as Mosul to hit discrete targets, and to minimize damage and civilian casualties, the commander said, those types of attacks would require specially trained U.S. troops to identify the target and direct pilots to it.

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