Two Americans were among the 31 people who died in terror attacks on the Belgian capital Tuesday, according to multiple reports.

Citing unnamed sources, NBC news said the families of the two American victims were still being notified of their deaths. They were not identified and it was not immediately clear where they were killed.

The news came as Secretary of State John Kerry visited Brussels to share America's condolences for Tuesday's attacks.

After a meeting with Belgian Prime Minister Charles Michel, Kerry said the "United States is praying and grieving with you for the loved ones of those cruelly taken from us, including Americans, and for the many who were injured in these despicable attacks," the Associated Press reported.

Suicide bombers struck Brussels Airport and the Maelbeek Metro station Tuesday, killing 31 and injuring 270 others, according to Belgian officials.

At least four Americans have been reported missing in the wake of Tuesday's attacks including a married couple and a brother and sister living in New York. Three American missionaries were injured in the explosions at Brussels Airport.

Few victims have been identified. Family members confirmed Adelma Tapia Ruiz, a Peruvian mother of 3-year-old twins who lived in Belgium for years, died in the attack on Brussels Airport. On Friday, officials in the United Kingdom confirmed IT worker David Dixon was among the deceased. The Chinese embassy confirmed one Chinese national died.

On Thursday, prosecutors announced six people had been detained in raids connected to the bombings. More information on those detained and the possible charges they may face was not immediately available.