UPDATE: The pilot of a helicopter is dead after a crash-landing on top of a building in the heart of New York City sent smoke streaming from the midtown Manhattan roof.

The crash occurred atop 787 Seventh Ave., a 54-story building near 51st Street, officials said.

The pilot is the only presumed victim, New York City Mayor Bill De Blasio told reporters at an afternoon news conference.

"There is no indication that this was an act of terror and there is no ongoing threat to New York City based on the information we have right now," De Blasio said.

New York City Police Department Commissioner James O'Neill said the helicopter had taken off from the heliport on the East River at 34th Street at 1:32 p.m., 11 minutes before it slammed into the building.

Multiple senior law enforcement officials told NBC News it was slated to fly by or to the Statue of Liberty, but encountered difficulty that led to the crash.

“To go into that area, a helicopter would need the approval from LaGuardia Tower,” De Blasio said. "And we need to find out if that happened or not here. We do not know at this point."

Paul Dudley, manager at the helicopter's home base of Linden Municipal Airport in Linden, New Jersey, identified the pilot as Tim McCormack, who he said "was a highly experienced, well regarded commercial pilot who was flying in this area for many years."

"Either the weather or something mechanical had to overwhelm him," Dudley told NBC News.

"I believe he went to that rooftop because he knew he couldn’t go to the ground," Dudley said. "I believe he picked the rooftop to try to spare everyone else."

The cause of the crash is still unknown. Weather conditions were rainy and visibility was little more than a mile Monday, with cloud ceiling at about 500 feet.

O'Neill said the NYPD is working with the Federal Aviation Administration and the National Transportation Safety Board to determine what caused the helicopter, an Agusta A109E, to crash.

New York Gov. Andrew Cuomo said initial reports of the incident were unsettling.

“If you’re a New Yorker, you have a level of PTSD from 9/11 and I remember that morning all too well," Cuomo, who was at the scene, said, referring to post-traumatic stress disorder. "So, as soon as you hear an aircraft hit a building, I think my mind goes where every New Yorker’s goes."

A White House spokesperson said, "The president has been briefed on the helicopter crash in Manhattan and continues to monitor the situation."

There did not appear to be any debris knocked off the roof as a result of the crash. The site of the crash, the AXA Equitable Center, is more than 750 feet tall and was built in 1985.

Emergency responders flooded the area and desperately tried to clear midtown traffic to allow firetrucks and ambulances to go through.

“I’m calm because when I came down, I didn’t know what was going on,” said Electra Steward, whose employer, BNP Paribas, is in the building. “Had I known it was a helicopter crash at the time, I would have been way more panicked.”

Natali Mendes, who works on the 31st floor, told NBC News the floor bounced under her feet.

“We just heard a big tremor …. the building bounced, literally the floor bounced under our feet,” Mendes said. "We jumped almost."

There were some nervous moments as workers were evacuated and ran into a traffic jam on the stairwell, she said.

“We were trying to go downstairs and there was such blockage, so we had to take the elevator because the stairs were not moving," Mendes said. “Our security … was very efficient. Within minutes they got us out. It’s just that the stairs took forever and that was a bit concerning.”

Former NYPD Commissioner Bill Bratton told MSNBC the accident was "not as bad as it might have been."

Buildings in midtown Manhattan are not equipped for helicopter landings, he said.

“Rooftops in New York City have all types of control systems, etc., on them, and they’re not flat surfaces like they are in Los Angeles, where by law, tall buildings in Los Angeles have to have a landing pad on them," he said. “There is almost no building in New York that is capable of accepting a helicopter on it."

Last month, a helicopter crashed into the Hudson River shortly after taking off from a Manhattan heliport. The pilot, Eric Morales, was re-positioning his aircraft after refueling. He was treated for minor injuries to his hand.