Obama: Boston attacks 'cowardly' act of terror

US President Barack Obama on Tuesday branded the Boston bombings a "cowardly" act of terror, but said it was still unclear if a foreign or domestic group or individual was behind the attacks.

"This was a heinous and cowardly act," Obama said at the White House. "Any time bombs are used to target innocent civilians, it is an act of terror."

Obama said while the impact of the attacks near the finish line of the Boston marathon on Monday, which killed three people and wounded more than 170 others, were clear, the motives and the identify of those responsible was not.

"What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual," he said.

But the president again vowed to bring whoever was behind the assault to justice, and warned that America would not be cowed by terrorism.

"We also know this -- the American people refuse to be terrorized," he said.

In frank and direct language, Obama vowed to keep Americans up to speed with developments in the investigation and asked them to remain vigilant.

"What I have indicated to you is what we now know. We know it was bombs that were set off. We know that obviously they did some severe damage. We do not know who did them," he said.

"We don't have a sense of motive yet. So everything else at this point is speculation."