White House labels Boston blasts as 'act of terror

US authorities do not yet know who planned and carried out explosions at the Boston marathon Monday that killed three and injured over a 140 people, but they are handling the incident as "an act of terror," a White House official said.

"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.

Two bombs ripped through the crowd at the finish line of the Boston Marathon on Monday, killing three people and injuring over 140 people with some victims suffering amputations.

US President Barack Obama, however, was more cautious in his statements.

"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts," Obama said in a televised statement from the White House. "But make no mistake, we will get to the bottom of this, and we will find out who did this, we'll find out why they did this."

Obama said he has directed the federal government to increase security around the United States as necessary after the explosions.

It was the worst bombing on US soil since security was tightened after the attacks of September 11, 2001, and President Obama promised to hunt down whoever was responsible for the attack on a day when tens of thousands of spectators packed the streets to watch the world-famous race.

The FBI had taken lead in investigations through a Joint Terrorist Task as of late Monday, and was in the process of going through property that was discarded at scene.

No one had been arrested and the White House official said it would have to be determined whether the attack came from a foreign or a domestic source.

Boston police also reported another explosion at the John F. Kennedy Presidential Library, which is three miles from the marathon's finishing line.

"We are not certain that these incidents are related, but we are treating them as if they are," Boston Police Commissioner Ed Davis told a news conference.

Davis said that the blasts, apparently some 50-100 meters apart, caused "multiple casualties." He added urged people to stay home or go back to their hotels, and to avoid congregating in large crowds. "People should be calm, but they should understand that this is an ongoing event," he said. 

Two high-level US law enforcement officials said one or more bombs were responsible for the explosions.

"It sounded like a sonic boom. I haven't stopped shaking yet," said Melissa Stanley, who watched her daughter cross the finish line four minutes before the explosions.

A fireball rose from behind spectators and a row of flags, video posted on the New York Post website showed. Other pictures showed blood stains on the ground and several people knocked down.

Massachusetts General Hospital was treating dozens of victims of the explosion in its emergency room, six of them in critical condition, a spokeswoman said.

Hundreds of thousands of spectators typically line the 26.2 mile (42.19 km) race course, with the heaviest crowds near the finish line. The blasts occurred more than five hours after the start of the race, at a time when most top athletes were off the course but slower amateur marathoners were still running.

The Boston Marathon has been held on Patriots Day, the third Monday of April, since 1897. The event, which starts in Hopkinton, Massachusetts and ends Boston's Copley Square, attracts an estimated half-million spectators and some 20,000 participants every year