A bomb that tore through an upscale New York neighbourhood, injuring 29 people, was an act of terror, the state governor says, but the motive is unknown and there has been no claim of responsibility.
Heavily armed police and National Guard soldiers were deployed throughout New York as the city of 8.4 million prepared to welcome world leaders at the UN General Assembly on Monday.
The attack happened late on Saturday in Chelsea, one of Manhattan's most fashionable districts packed with bars, restaurants and luxury residential buildings.
"A bomb exploding in New York is obviously an act of terrorism, but it's not linked to international terrorism," New York Governor Andrew Cuomo told reporters on Sunday as he visited the crime scene.
"In other words, we find no ISIS connection, etcetera," said Mr Cuomo in reference to the Daesh terrorist organization, which is based in Iraq and Syria.
Police discovered a second bomb planted four blocks away, which was safely defused and is currently being analysed, officers said.
The bombing came as a Daesh-linked news agency claimed their "soldier" carried out a stabbing attack in a U.S.. mall that left nine people injured late Saturday in the state of Minnesota.
Local police said the attacker "made some references to Allah", but the motive was unclear.
The attacker was shot dead by an off-duty police officer. There was no suggestion it was linked to the New York bombing.
Police pour over evidence
New York police chief James O'Neill said no individual or group had yet claimed responsibility.
Mr O'Neill said he could not say with a "100 per cent degree of certainty" where the blast originated.
U.S. media reported that it was planted in a dumpster on 23rd Street where major construction work is taking place.
A hot line has been set up for tips. Police have a video of the bombing and were searching for anyone seen in the area before the explosion.
"We know it's a very serious incident, but we have a lot more work to do to be able to say what kind of motivation was behind this," Mayor Bill de Blasio told reporters.
"Was it a political motivation, a personal motivation. What was it? We do not know that yet," he added, calling on residents to be vigilant.
FBI official William Sweeney said federal investigators would be pouring through online traffic, individuals and organisations.
"We will look at everything," he told the news conference.
"We'll look at social networks, at all the incoming tips and leads. Everything that comes in gets a look. We don't discard anything."
New York will see a stepped up security presence, with an additional nearly 1,000 state police and National Guard deployed to airports, bus terminals and subway stations, officials said.
The explosion on 23rd Street, between Sixth and Seventh Avenues, caused "significant" damage, shattering glass and shrapnel across the street, but there is no structural damage to any buildings.
Heavy weapons teams among beefed up security
While the two devices planted in Manhattan appear to be similar, they seem to be different than a pipe bomb that exploded in the neighbouring state of New Jersey on Saturday causing no injuries, officials said.
Of the 29 people who sustained injuries in New York, 24 were taken to hospital with various degrees of scrapes and abrasions from glass and metal. All have since been released, officials confirmed.
New York lauds itself as the safest big city in America. Violent crime is rare in Manhattan and police say they have foiled 20 terrorist plots since the 2001 Al Qaeda hijackings destroyed the Twin Towers.
New York Congressman Peter King said the fact that officials had not yet determined a known terrorist link was not necessarily conclusive.
"In many of these cases we don't know until two, three or four days later whether or not there is a terrorist link," he told CBS.
"The fact there is no evidence right now doesn't mean much."
Police have sealed off northern Chelsea around the crime scene and dozens of officers were out in force Sunday.
"Today there special events occurring throughout the city. Actually in all five boroughs. We've increased our police presence in each of these events," senior New York police officer Carlos Gomez said.
"We've also added more counter-terrorism officers as well as heavy weapons teams at some of these events.
"Teams from the strategic response group as well as the critical response command."
