06:17

Obama says mobilizing resources to probe into Boston explosions.

Obama says to get to the bottom of Boston blasts, bring all responsible to justice.

Obama says increase of security in US is necessary.

05:47

WASHINGTON - Police said on Monday that a third explosion hit the Boston JFK Library, but it remains unclear whether it was related to the previous two blasts near the finish line of the Boston marathon.

"There was a third explosion at the JFK library," Boston Police Commissioner Ed Davis said at a press conference, adding that he was not yet aware of any injury in the third explosion.

Davis announced a police tipline for information that could lead to the arrest of those who were responsible for the attacks. He also advised people to stay at home or in the hotel.

Meanwhile, US media reports said that the number of those injured in the explosions has soared to over 100, with at least two deaths.

03:27

WASHINGTON - At least two were dead, 28 injured in two explosions taking place near the Boston Marathon finish line on Monday afternoon, US media reported.

CNN still image taken from video shows the scene of an explosionat the 117th Boston Marathon in Boston, Massachusetts, US, April 15, 2013. [Photo/Xinhua]

President Barack Obama has been notified of the incident in Boston, a White House official told White House pool media.

Obama directed his administration to provide whatever assistance necessary in the investigation and response, said the official.

Intelligence officials said two more explosive devices have been found at Boston Marathon before being dismantled, media here reported.

The organizers of the Boston Marathon said on Facebook page that bombs caused the two explosions.

The Boston police bomb squad were sweeping the area for suspicious packages, ABC reported. Law enforcement authorities also told ABC they were testing smoke and materials at the scene for chemical substances.

The two explosions happened at around 3 pm and went off within seconds of each other. The first runners in the marathon crossed the finish line around noon.

The Boston Police Department said two explosions happened on the Boyleston street near the finish line. Local police, fire and bomb squad have arrived at the scene responding to the incident.

A man in a wheelchair is taken from the scene after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. [Photo / Xinhua]

A photographer with Fox affiliate WFXT reported a number of victims with lost limbs.

CNN's video showed heavy smoke on the street, while emergency crews were treating the injured. Bleeding spectators were seen being carried to the medical tent set up to care for the runners.

Many of the injured appeared to be spectators who had gathered at the scene, local media WCVB.com reported. The area has been evacuated.

Police in New York and Washington D.C. have also beefed up security around the cities in the wake of explosions.