President Barack Obama called the Boston bombings an "act of terror'' on Tuesday, but said it is not clear yet whether the twin blasts were the work of a foreign or domestic group or a "malevolent individual.''

In an appearance in the White House briefing room, Obama urged Americans to be vigilant and to watch for suspicious activity a day after two explosions rocked the finish line of the Boston Marathon, killing at least three people and wounding scores more.