(CNN) Defense Secretary Ash Carter said Thursday that the Pentagon is looking into an incident in which U.S. support jets assisting Syrian rebels diverted to Iraq to strike suspected ISIS militants last week, leaving the rebels without badly needed air cover.

Speaking to reporters on a flight to Warsaw, Poland, for a NATO meeting, Carter said the review will "see if there are any lessons learned."

One aircraft flying air support missions over Al-Bukamal during the recent fighting there was called off and diverted to strike at a massive convoy of suspected ISIS militants that had been observed to be trying to leave the southern outskirts of Fallujah, according to a senior U.S. official. Several aircraft were diverted from various patrols in different parts of Iraq, but only one was taken away from the Al-Bukamal battle.

Carter said the U.S.-backed fighters have moved back in since being pushed back by ISIS. "Those forces have regrouped and are back in the fight," Carter said. He added that they didn't anticipate how quickly things would develop in Fallujah. "In this case, they were adjusting to another very successful set of airstrikes in Fallujah," he said.

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