The White House does not yet know who planned and carried out explosions at the Boston marathon but is handling the incident as "an act of terror," a White House official said on Monday.

"Any event with multiple explosive devices - as this appears to be - is clearly an act of terror, and will be approached as an act of terror," the official said.

"However, we don't yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic," the official said.