"We still have not accounted for every official US government employee or their family members on the ground in Brussels," Toner said.
Earlier in the day, the State Department said at least 12 Americans were injured in Tuesday’s attacks at the Brussels international airport and a metro station, but said it was not aware of any US citizen deaths.
Daesh group has claimed responsibility for the attacks, which killed at least 31 people. Belgian police are still searching for fugitive suspect Najim Laachraoui.
On Friday, US Secretary of State John Kerry will travel to Brussels, where he will discuss efforts to counter violent extremism with Belgian and European Union officials.