NEW YORK (NYTIMES, REUTERS) - A would-be suicide bomber set off a pipe bomb strapped to his body during the morning rush hour on Monday (Dec 11), deep inside one of Manhattan's busiest commuter corridors, police said.

He chose the location because of its Christmas-themed posters, recalling strikes in Europe against Christmas markets, he told investigators, and set off the bomb in retaliation for US air strikes on Islamic State in Iraq and Syria (ISIS) targets in Syria and elsewhere, several law enforcement officials said.

The blast echoed through the subway tunnels just off Times Square, and filled parts of the Port Authority Bus Terminal with smoke as commuters fled into the streets to escape what the authorities quickly concluded was an act of terror.

A suspect, identified by the police as Akayed Ullah, 27, an immigrant from Bangladesh who lived in Brooklyn, was in police custody.

He suffered burns to his hands and abdomen, and was in serious condition at Bellevue Hospital Centre, according to Daniel A. Nigro, commissioner of the New York Fire Department. Four other people had minor injuries, he said.

Ullah is from the Bangladeshi city of Chittagong. He had no criminal record there and last visited Bangladesh on Sept 8, said the police. He had a black cab/limousine driver's licence from 2012 to 2015, after which it expired, the New York Taxi and Limousine Commission said.

Talking to The Daily Star, Shamim Ahsan, Consul General at Bangladesh Consulate in New York, said that they came to know from the media that the suspect was a Bangladeshi. He said he was in touch with the authorities and was awaiting confirmation of the attacker's identity.

The attack, at 7.20 am, occurred in a long pedestrian walkway connecting the Eighth Avenue, Seventh Avenue and Broadway subway lines. Walking among the commuters trudging beneath Times Square was a man in a hooded sweatshirt. Then a deafening boom - from him - and then smoke. Then everyone ran.

Ullah had attached the pipe bomb to himself with a "combination of Velcro and zip ties", said James P. O'Neill, commissioner of the New York Police Department. The secure fastening may have indicated that Ullah entered the subway intending to carry out a suicide bombing.

New York state Governor Andrew Cuomo, speaking at a news conference near the site of the explosion, described the device as "amateur-level".

At a news conference on Eighth Avenue just outside the Port Authority, police displayed a picture of Ullah that appeared to have been taken inside the subway walkway after the blast. In it, he is curled in a foetal position; his exposed stomach is blackened.

Ullah acted alone, Mayor Bill de Blasio said, adding that no other devices had been found.

Cuomo told CNN the explosive in the pipe ignited, but the pipe itself did not explode. "So he wound up hurting himself; several others in the vicinity."

He said the attacker apparently used the Internet to obtain information on how to make a bomb. At the news conference, De Blasio said the incident, which happened at the start of rush hour, was "an attempted terrorist attack".

"As New Yorkers, our lives revolve around the subways. When we hear of an attack in the subways, it is incredibly unsettling," de Blasio said.

"The choice of New York is always for a reason, because we are a beacon to the world. And we actually show that a society of many faiths and many backgrounds can work.

"The terrorists want to undermine that," the mayor added. "They yearn to attack New York City."

De Blasio spoke within hours of the attack. But the investigation by the Joint Terrorism Task Force was still in its preliminary stages.

New York City was a target, said John Miller, deputy police commissioner of intelligence and counterterrorism. He cited the attacks of Sept 11, 2001, that killed more than 2,750 people in New York and nearly 3,000 people in total; and the World Trade Centre bombing of Feb 26, 1993, which killed six people.

Fox News reported that the attacker made the device at his job at an electrical company and there were no known co-conspirators.

A pro-Islamic State media group, Maqdisi Media, portrayed the attempted terror attack as a response to US President Donald Trump's recognition on Wednesday of Jerusalem as Israel's capital, according to the Site Intelligence Group. Site tracks and analyses online activity by extremist groups.

However, CNN, citing a law enforcement source, reported that Ullah told investigators recent Israeli actions in Gaza were the reason he carried out the attack.

The incident occurred less than two months after an Uzbek immigrant killed eight people by speeding a rental truck down a New York City bike path, in an attack for which the ISIS claimed responsibility.

In September 2016, a man injured 31 people when he set off a homemade bomb in New York's Chelsea neighbourhood. The incident was captured on security video, police said.

Christina Bethea was in the underground walkway, headed to her job as a security guard, when Monday's explosion nearly knocked her over, sending a haze of smoke into the corridor packed with commuters. She did not see where it came from, she said.

"As soon as we heard 'boom', we began to run," she said. An hour after the attack, she stood outside the Port Authority, calling her mother and father in North Carolina to tell them she was OK. "I feel good," she said. "I am alive!"

The authorities were searching Ullah's residence on Ocean Parkway, pursuant to a federal warrant, one law enforcement official said.

While no formal announcement had been made, both federal and local law enforcement officials indicated that Ullah would be prosecuted in federal court in Manhattan by the office of the acting US attorney for the Southern District of New York, Joon H. Kim.

The attack is being investigated by the Joint Terrorism Task Force, which is made up largely of Federal Bureau of Investigation agents and New York detectives, along with investigators from a score of other federal, state and local law enforcement agencies.

The attack roiled commutes across the region. All subway lines were directed to skip 42nd Street stops, according to the Metropolitan Transportation Authority. By late morning, only the A, C and E were still skipping the stop. The Port Authority was evacuated for several hours; it reopened around 10.30am.

All morning, thwarted travellers spilled into the streets of Times Square, towing suitcases in bewildered silence. They gathered at police cordons stretched across 42nd Street, filming a scene of organised chaos as scores of emergency vehicles arrived at the scene every few minutes.

John Frank, 54, was standing on 42nd Street by the Port Authority exit when he felt tremors through the pavement. "That's how strong it was," he said. Everyone began to run.

He stood on Eighth Avenue a few blocks away on Monday morning, shaken, leaning on a garbage pail for support. "In New York City, we are vulnerable to a lot of things," he said. "These incidents are happening too frequently."

At the White House, spokesman Sarah Sanders told reporters: "This attack underscores the need for Congress to work with the President on immigration reforms that enhance our national security and public safety."

" ...We must ensure that individuals entering our country are not coming to do harm to our people," she said during a regular news briefing. "And we must move to a merit-based system of immigration."

More than 200,000 people use the Times Square station, the city's busiest, each weekday, according to the Metropolitan Transportation Authority. Ten train lines use the station. The bus terminal is the busiest in the US, according to the Port Authority.

On a typical weekday, about 220,000 passengers arrive or depart on more than 7,000 buses. The bus terminal is just adjacent to the subway station's western section.

A long, narrow underground tunnel connects that part of the station to its eastern section, and is used by thousands of commuters during rush hour. Buskers and other entertainers at entrances to the tunnel often draw crowds.

The incident rippled through American financial markets, briefly weakening stock markets as they were starting trading for the week and giving a modest lift to safe-haven assets such as US Treasuries.

Technology and energy stocks gained in early afternoon trading on Monday, helping Wall Street shake off uncertainties following an explosion.