A New York City police officer is among four people killed in a shooting inside a Midtown Manhattan high-rise.

Police said the gunman walked into the office building on Monday evening with an MR rifle in clear view before spraying bullets in the lobby and on an upper floor. He then took his own life.

The shooting, in the centre of the most populous US city, sent pedestrians running for cover as police warned of an active gunman around 1830 ET. Those inside the building cowered as gunfire echoed - some even piling couches and chairs against doors as they hid in offices.

Police say the suspect, Shane Devon Tamura, 27, travelled from Las Vegas, but a motive is unclear.

Among those killed was New York City officer Didarul Islam, who was working security at the building at the time of the attack. The three other victims have yet to be identified.

New York City Police Commissioner Jessica Tisch said the suspect entered the building at 345 Park Avenue, which houses businesses including the National Football League (NFL) and the investment management company Blackstone, and immediately opened fire.

He fatally shot Mr Islam in the building's lobby, then took aim at a woman who hid behind a pillar. He proceeded through the lobby, "spraying" it with gunfire, hitting multiple people, including a security guard hiding behind a desk and another man nearby.

As he waited for an elevator, a woman stepped off, but the gunman spared her and let her pass, Commissioner Tisch said.

He made his way to the 33rd floor - where Ruden Building Management is located - and again opened fire - killing one person before walking down a hallway and shooting himself in the chest, she added at a Monday night news conference.

Authorities are unsure of a motive, though Tisch noted the gunman had "documented mental health history."

Tamura travelled from Las Vegas to the East Coast over the last few days, stopping in Colorado, Nebraska, Iowa and New Jersey. He arrived in New York shortly before the shooting. It's unclear whether he has any ties to New York, the building or Ruden Building Management.

Police searched the suspect's BMW, which he had double-parked out front, and found multiple magazine rounds and a revolver. The vehicle was registered in Nevada to Tamura, who was a registered gun owner in the state.

Local officials and authorities celebrated Officer Islam Monday night. "He died as he lived - a hero," Ms Tisch said.

An officer with the department for three years, Officer Islam was an immigrant from Bangladesh. He and his wife have two children, and his wife is currently pregnant with their third child.

"He was doing the job that we asked him to do," Ms Tisch said. "He made the ultimate sacrifice."

The incident brought parts of Midtown Manhattan and public transportation to a halt. A BBC journalist at the scene reported seeing scores of police vehicles and at least one person with a bloodied chest being taken away on a stretcher.

Bystanders reported hearing what sounded like gunshots, and police told those in the area, including the BBC journalist, to shelter in nearby buildings.

Police worked floor-by-floor to clear the building, an effort that took hours.

Mama Bouhenni, working at the nearby Sip & Scoop coffee shop as the incident unfolded, told the BBC she saw dozens of people from surrounding buildings being evacuated.

They held their hands above their heads, she said.

Ms Bouhenni said she recognised many of them as customers. "This is so terrifying for them. I hope they can go home safe."