" First on CNN: American killed as hostages rescued in Iraq, sources say " See Video (CNN)An American was killed in an operation involving the U.S. military to rescue dozens of hostages who "faced imminent mass execution" in Iraq, the Pentagon said Thursday.

About 70 hostages, including more than 20 members of the Iraqi Security Forces, were liberated in the operation, which involved U.S. special operations troops as well as Kurdish and Iraqi forces, U.S. officials said.

"There was not a lot of time," one U.S. official told CNN on condition of anonymity Thursday. "The threat of execution was imminent."

The firefight represents the first time U.S. forces stepped into combat against ISIS in Iraq, the U.S. official said.

The American troops, who were on an advise-and-assist mission, stepped in after Kurdish Peshmerga soldiers came under fire, the official said.

One U.S. service member was mortally wounded -- the first American combat death in Iraq since November 2011 -- during the rescue mission at a prison of the Islamist militant group ISIS near Hawija in the northern province of Kirkuk, the Pentagon said. Four Peshmerga soldiers were wounded.

There were no Kurds among the hostages, the Kurdistan regional government said in a statement. The U.S. had claimed earlier that about 70 Kurdish hostages were rescued.
Who are the Kurds?

Who are the Kurds? 01:57

In a statement, Gen. Lloyd J. Austin III, commander of the U.S. Central Command, described the mission as "complex and highly successful."

"We commend and congratulate the brave individuals who participated in this successful operation that saved many lives, and we deeply mourn the loss of one of our own who died while supporting his Iraqi comrades engaged in a tough fight," he said. "Our gratitude and heartfelt condolences go out to this young man's family, his teammates and friends."

Further details about the situation, including the identities of the hostages and hostage-takers, weren't immediately available.

The operation came more than a month after Kurdish security forces said the Islamist militant group ISIS kidnapped dozens of men near Hawija. It wasn't immediately clear whether the overnight rescue was related to this early September kidnapping.

The Kurds are an ethnic group with an autonomous region in northern Iraq. The Kurdish Peshmerga, the military force that protects this region, has been fighting ISIS, which captured parts of Iraq and Syria for what it calls its Islamic caliphate.