The United States lacked specific intelligence warning of Tuesday's attacks in Belgium but strongly believed that such a strike was possible, particularly after Belgium's arrest last week of a key suspect in the Paris attacks, US officials said.

Thirty-four people were killed in blasts at Brussels airport and in a rush-hour metro train in the Belgian capital, triggering security alerts across Europe and bringing some cross-border traffic to a halt.

The attacks came just four days after the arrest of French citizen Salah Abdeslam - the prime surviving suspect in November's Paris attacks on a stadium, cafes and a concert hall - who was captured by Belgian police after a shootout on Friday.

Three US government officials acknowledged that the United States believed an attack by Islamic State in Brussels was possible, if not likely. Still, they were not aware of any US intelligence about where or when the attack would occur.

One of the main US lines of inquiry is that even though the attack may represent retaliation for the arrest of Abdeslam, it was likely already in the works before his arrest.

Under that scenario, the attack date was already on the schedule before his arrest, and possibly advanced somewhat because of his arrest, two of the officials said.