Three separate attacks carried out in the United States on a single day -- a New York bombing, a Minnesota mass stabbing and a New Jersey pipe bomb blast -- were under investigation Sunday for potential terror links.

Authorities say there is no evidence that any of the attacks were coordinated but the timing in less than 24 hours raises fears about security -- already a hot button issue in the country's deeply divisive presidential election between Hillary Clinton and Donald Trump.

Twenty-nine people were injured when a bomb exploded in New York's upmarket Chelsea neighborhood on Saturday night, damaging neighboring buildings, shattering glass and sending shrapnel flying across the street.

A second bomb was uncovered by police four blocks away and defused safely, before undergoing analysis.

Hours earlier, less than 100 miles (160 kilometers) south in New Jersey, a pipe bomb exploded in a trash can on the sidelines of a Marine Corps run, causing no injuries but forcing the cancellation of the race.

More than 1,200 miles away to the West, an assailant reported to be Somali-American went on a stabbing spree in a mall in St. Cloud, Minnesota, injuring nine people before being shot dead by an off-duty police officer.

US authorities said the motive of all three attacks was unclear, but elected officials quickly identified them as terror-related.

"If you look at a number of these incidents, you can call them whatever you want -- they are terrorism, though," New Jersey's Republican Governor Chris Christie, a key member of the Trump campaign, told CNN.

"A bomb exploding in New York is obviously an act of terrorism, but it's not linked to international terrorism," State Governor Andrew Cuomo told reporters on Sunday after touring the scene of the explosion.

"In other words, we find no ISIS connection, et cetera," said Cuomo in reference to the Islamic State group. But he also stressed the lack of international terror link was preliminary.

There has been no claim for the bombings in Manhattan or New Jersey, but a jihadist-linked news agency claimed that an IS "soldier" carried out the Minnesota stabbing.

IS has repeatedly called for attacks on countries in the US-led coalition bombing the extremist group in Syria and Iraq.

"This should steel our resolve to protect our country and defeat ISIS and other terrorist groups," said Clinton, whose lead in the polls has recently taken a dip, condemning what she called "apparent terrorist attacks."

Trump, who called the explosion in New York a bombing more than 12 hours before officials or police did so in public, tweeted his "best wishes and condolences to all of the families and victims of the horrible bombing."

In Minnesota, FBI agent Rick Thornton confirmed that federal agents were investigating the stabbing as "a potential act of terrorism," as local media identified the suspect as a 22-year-old Somali-American.

Police confirmed that the assailant asked some victims whether they were Muslim before attacking them and made "references to Allah."

While all 29 people who were injured in the New York bombing have been released from medical care, three of those hurt in Minnesota remain in hospital, officials said.

US media reported that the New York bomb was planted in a dumpster, but police say they are still not "100 percent" certain where the blast originated with major construction work taking place close by.

"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," he added, calling on residents to be vigilant.

Fifteen years after the September 11, 2001 attacks, officials stress that the United States is safer from terror plots that originate from overseas but more at risk from the lone-wolf attack perpetrated by individuals who may be inspired by IS or Al-Qaeda propaganda.

New York went on alert, deploying nearly 1,000 extra state police and National Guardsmen to airports, bus terminals and subway stations as the city prepared to host world leaders at the UN General Assembly from Monday.

New York lauds itself as the safest big city in America. Violent crime is rare in Manhattan and police say they have foiled 20 terror plots since the 2001 Al-Qaeda plane hijackings destroyed the Twin Towers.

"In many of these cases we don't know until two, three or four days later whether or not there is a terrorist link," New York Congressman Peter King told CBS.

"The fact there is no evidence right now doesn't mean much," he added.