WASHINGTON (AP) - Defence Secretary Chuck Hagel said on Tuesday that the deadly twin bombings at the Boston Marathon finish line was a "cruel act of terror" and vowed that those who are responsible will be brought to justice.

Testifying on Capitol Hill, Hagel was the first Obama administration official to reference terror or terrorism after the bombings on Monday killed three and wounded more than 140 people on Monday afternoon.

President Barack Obama, in his own brief statement at the White House late on Monday, made no mention of terrorists or terrorism as a possible cause of the bombings. A White House official speaking on condition of anonymity because the investigation was still unfolding did say the attack was being treated as an act of terrorism.

Hagel said any event in which explosive devices are used is clearly an act of terror.

"As the president said yesterday, we still do not know who did this and why, and a thorough investigation will have to determine whether it was planned or carried out by a terror group, foreign or domestic," Hagel told the House Appropriations defence subcommittee.

He mentioned the Pentagon's connection to the race, with many in the defence community participating in the race and commended the quick work of the Massachusetts National Guard to assist after the explosions.

Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, also praised the National Guard. Dempsey was testifying with Hagel.

Hagel said the thoughts and prayers of those at the Pentagon are with the people of Boston.