NEW YORK CITY: A former employee has smuggled a firearm under his lab coat and opened fire inside the Bronx Lebanon Hospital in New York City, killing a female doctor and leaving several other doctors fighting for their lives.

Witnesses said staff were scrambling to move patients to safety behind closed doors, locking themselves in bathrooms with the help of dozens of “hero” nurses and doctors.

Police received a 911 call at 4pm on Friday about a gunman opening fire inside the hospital.

Heavily-armed police evacuated 100s of terrified staff and visitors before searching the 1000-bed hospital floor by floor where they discovered another six people had also been reportedly shot, three in a critical condition.

Police said the gunman was found dead inside the building. He had also reportedly tried to set himself on fire.

Sky News reported one witness saying the gunman walked into the hospital with the firearm and “knew exactly where he was going”.

“I thought I was going to die,” said Renaldo Del Villar, a 55-year-old patient who was in the third-floor emergency room getting treatment for a lower back injury.

Law enforcement officials identified the shooter as Dr Henry Bello, who was described on the hospital website as a family medicine physician.

He resigned from the hospital in 2015 after some sort of dispute, according to one law enforcement official, who said Bello used an AR-15 in the attack.

The officials were not authorised to discuss the still-unfolding investigation and spoke on condition of anonymity.

“This was a horrible situation unfolding in a place that people associated with care and comfort, a situation that came out of nowhere,” said Mayor Bill de Blasio, adding that one female doctor had been killed and six others wounded.

The FBI tweeted that they had found “no nexus to terrorism” so far.

Employees and their loved ones described the horrifying moments as they scrambled for information.

Garry Trimble said his wife, hospital employee Denise Brown, called him from inside the hospital shortly after 3pm.

“She woke me up and told me there was a situation, somebody’s out there shooting people,” Trimble said as he waited for his wife to come out. “I could hear in her voice she was shaking and about to cry.”

Gonzalo Carazo described the scary scene to WCBS-TV. “I saw one of the doctors and he had a gunshot wound to his hand,” Carazo said.

“All I heard was a doctor saying, “Help, help!”

Carazo locked himself in a room for about 15 minutes until police came and led him out of the hospital.

The 120-year-old hospital is one of the busiest emergency rooms in New York City and is located 2.4km north of Yankee Stadium.

In 2011, two people were shot at Bronx Lebanon in what police said was a gang-related attack.

—with AAP