Three explosions rocked the US city of Boston on Monday afternoon, killing two and injuring over 100, turning the traditional local marathon festivity into a scene of blood and terror.

While President Barack Obama remained cautious and avoided calling the blasts a terror attack, some US media and experts already started to use the phrase of "terrorist bombings."

If confirmed by further investigations, this could be the first major terror attack that has actually hit the US soil since 9/11, though authorities in the past decade have foiled dozens of terrorist plots.

SHADOW OF TERROR

In a live televised statement from the White House, Obama acknowledged that his government has not yet got "all answers" about the Boston explosions, but vowed to hold all those behind them accountable.

Meanwhile, he said the government has taken steps to beef up security across the country "as necessary."

Alarmed by the Boston blasts, all major US metropolises, including Washington D.C., New York, Los Angeles, Chicago, San Francisco and Miami, have tightened up security measures.

The US Secret Service reportedly expanded its security perimeter at the White House following the Boston explosions.

Meanwhile, a New York Police Department (NYPD) spokesman said that police have beefed up security at hotels, landmarks and other strategic locations, as well as critical infrastructure such as subway stations, as a precaution "until more about the explosions in Boston is learned."

"We have 1,000 members of the NYPD assigned to counter- terrorism duties, and they -- along with the entire NYPD and the investments we have made in counter-terrorism infrastructure -- are being fully mobilized to protect our city," Mayor Michael Bloomberg stated.

Even in London of Britain, where a marathon is scheduled for this coming Sunday, the Metropolitan Police have been reviewing their security plan for the race after the Boston explosions.

"IT JUST HITS HOME"

"You put in all that hard work and it's such a positive day ... and to have something like this happen, it takes all the fun out of it. It's the best marathon in the world. And also, my brother was a National Guard in Afghanistan three years ago. So it just hits home, something like that," Erica Castenzo, a Boston resident, told Xinhua shortly after the blasts in town.

Hannah Truslow, an eyewitness from Portsmouth, New Hampshire, recalled: "We were in the VIP seating and we saw the first bomb go off ... and people blow up in the streets and going down ..." She burst into tears before she could finish the sentence.

Boston police confirmed later on Monday that two of the explosions happened at around 2:50 p.m. near the finish line of the race, going off within seconds of each other. The third one occurred at the Boston John F. Kennedy Presidential Library, but it remains unclear whether it was related to the previous two.

Media reports said that among the two dead was an 8-year-old child, whereas six of the 100-plus injured were in critical condition. Many of the injured appeared to be spectators who had gathered at the scene to watch the world's oldest modern marathon, which attracted over 20,000 runners this year.

Cao Yue, a student at the University of Boston, told Xinhua that he was just 200 meters away from the Boston Marathon finish line when the explosions went off.

Cao said he heard two huge explosions and saw heavy smoke on the street, which he originally thought to be some celebration of the event. But he soon found people screaming, crying and rushing to him from the finish line, many of them spectators.

"I was totally shocked and people were really scared," recalled Cao.

UNFORESEEABLE REPERCUSSIONS

The 9/11 terror attacks in 2001 have changed profoundly the security landscape for the United States and the rest of the world, followed by the US-led invasion of Afghanistan and a decade-long manhunt that ended with the US killing of terror mastermind Osama bin Laden in May 2011.

The Obama administration has been actively seeking American troops' early withdrawal from Afghanistan, and has cited the death of bin Laden as one of its major achievements in the war against terror.

For the Monday explosions, Boston police said that no suspect has been taken into custody. And so far no individual or organization has claimed responsibility.

President Obama, who was briefed by Homeland Security Adviser Lisa Monaco, FBI Director Robert Mueller and Secretary of Homeland Security Janet Napolitano on the investigation and response to the incident, said in his statement: "We still do not know who did this or why. And people shouldn't jump to conclusions before we have all the facts."

"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," the president stressed. "Any responsible individuals, any responsible groups will feel the full weight of justice."