An explosion in the Chelsea neighborhood of Manhattan that injured 29 people on Saturday night was an “intentional act,” but did not appear to be connected to terrorism, Mayor Bill de Blasio said.

Meanwhile, authorities early Sunday were investigating a pressure cooker with a cellphone and wires attached that was found several blocks away, warning nearby residents to stay away from their windows as they determined if it was a dangerous item, law-enforcement sources said.

De Blasio, speaking near the blast site and flanked by NYPD, FDNY and other officials, said injuries from the blast were “significant.” One person was seriously injured and 23 others were taken to hospitals for treatment after the 8:30 p.m. blast at 131 West 23rd St., authorities said.

None of the people injured was likely to die, the mayor said.

FDNY officials said most of the injuries were scrapes and abrasions from glass and metal, but the serious injury was a puncture wound.

The mayor also said the blast, which was still under investigation, did not appear to have a “terror connection.”

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Officials said they were backtracking through video taken in the area, including video that captured the blast itself, to determine who placed the device.

James P. O’Neill, on his first full day as NYPD commissioner, said “the exact cause of this explosion has not been determined,” and added that there was “no evidence of a gas explosion.”

De Blasio also said there did not appear to be a connection to a blast earlier in the day in a trash can in Seaside Park, New Jersey.

“There is no specific and credible threat against New York City at this point in time from any terror organization,” he said.

A Twitter user shows the view outside an apartment at 23rd and 6th in the Chelsea neighborhood of Manhattan, following the scene of a possible explosion on Saturday, Sept. 17, 2016. Photo Credit: @nnehajainn / Twitter

The second site — on West 27th Street between Sixth and Seventh avenues — was being actively investigated as the mayor spoke. Police were telling residents on that street to stay away from windows until the scene was cleared.

In 2013, two pressure-cooker bombs filled with nails and ball bearings exploded at the Boston Marathon, killing six people and injuring 280 others.

“Whatever the cause, whatever the intentions here, New Yorkers will not be intimidated,” de Blasio said on Saturday.

Gov. Andrew M. Cuomo issued a statement Saturday night saying “full state resources have been made available” to the city, and White House officials said President Barack Obama had been “apprised of the explosion.”

Nassau police said on Saturday night they were on “heightened alert” and had intensified patrols in the county in response to both bombings.

Nearby residents reported hearing the Chelsea explosion, and police cleared sidewalks and streets of pedestrians and cars and pushed people back from the scene, where glass could be seen strewed around the street.

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FDNY officials said they were assessing surrounding structures for damage.

The NYPD counterterrorism unit on Saturday night released a photo of a dumpster twisted and crushed from the blast.

“Please clear the streets. Call it an evening,” an officer announced over a megaphone.

The blast occurred near the Visions at Selis Manor, a home for people with blindness, and the Townhouse Inn of Chelsea. A call to Visions facility was not answered Saturday night, and a man who answered the phone at the Townhouse Inn said he hadn’t been told anything about the investigation.

The ATF explosives task force also responded to the scene — a busy street filled with restaurants, bars and hotels.

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Lisa Budihas, 44, from Williston Park, was on her way to visit friends on 27th street when she realized what had happened hours earlier.

“I’m shocked right now,” she said, sitting on the median dividing Broadway. “For a second I said wait, what if something happens again? We’re in the middle of everything.”

But she said she doesn’t feel unsafe. Cops, she added “are all around us.”

Laurie Mechali, 48, was staying at a hotel on 27th Street. Mechali, from California, was in New York visiting with her family, who lives in Paris.

“We just came back and the street was shut down,” she said. The group had been walking around Times Square. “There’s a lot of police here, we don’t feel unsafe.

“There were attacks in Paris,” she added. “There’s not a place you can be safe.”

Jenny Vargas, 40, who lives about one block north, and her daughter Kayla, 7, were out to buy a pet fish when her husband, Kelvin, called and told her “there was an explosion.”

“I’m scared,” Kayla told her mother, who held her daughter’s hand as she carried a plastic bag of goldfish home.

Keith Allen, 58, of California, said he was on his way to dinner when he heard “a huge ka-boom. Everyone on the street just stopped and literally looked at each other.” Allen said he smelled sulfur, and within minutes he saw police cars starting to arrive.

Earlier Saturday, a pipe bomb exploded in a Jersey Shore town shortly before thousands of runners were due to participate in a charity 5K race to benefit Marines and sailors, authorities said.

No injuries were reported in the blast in Seaside Park around 9:30 a.m. Saturday, said Al Della Fave, spokesman for the Ocean County prosecutor’s office. He also said no surrounding structures sustained any apparent damage.

The third annual Seaside Semper Five run was canceled, and the immediate area was put on lockdown.

The pipe bomb was in a plastic garbage can when it exploded, Della Fave said. Authorities immediately cleared the boardwalk and beach area, and bomb-sniffing dogs were used to search the area.

Authorities said there was a report of at least one other explosive device that a New Jersey State Police bomb team would “make safe,” but further details were not disclosed.

With AP