An Air Force officer, his wife and four children were injured in the attack at the Brussels airport, U.S. officials said Tuesday, as more details began to surface about Americans hurt in the bomb blasts that killed at least 31 and wounded nearly 190.

The command also said that any official travel to the NATO hub would require approval. The restrictions don't apply to military personnel assigned to posts in the city.

European Command officials would not release details of the injuries to the Air Force officer or his family, due to privacy concerns. But U.S. officials confirmed the officer was a lieutenant colonel, and that he and his family were injured at the attacks in the airport. The officials spoke on condition of anonymity because they weren't authorized to discuss the details publicly.

Gen. Philip Breedlove, commander U.S. European Command, said the U.S. will assist authorities in Belgium in any way they can, adding that we "will continue to stand by our NATO allies and partners to defeat these terrorists who threaten our freedoms and our way of life."

European Command said that it is continuing to confirm the safety of U.S. military personnel and families in the region, including those who may be there on vacation or official travel.

The injuries bring the total to at least nine Americans who were hurt in the explosions that ripped through the Brussels Airport and a metro station Tuesday. Mormon church officials said three missionaries from Utah were seriously injured in the airport attack and have been hospitalized.

The Islamic State militant group has claimed responsibility for the attack.