A gunman believed to be Nigerian-born Henry Bello has killed a female doctor and injured six others after opening fire inside a hospital in Bronx, New York, US.

Bello killed himself after the shooting spree which took place on Friday on the 16th and 17th floors of the Bronx-Lebanon Hospital.

Bello is a former family medicine doctor at the hospital. He was reportedly asked after only six months on the job because of a s*xual harassment allegation levelled against him.

Maureen Kwankam, a doctor at the hospital, told New York Daily News that he promised to return and “kill” everyone when he was “fired” in 2015.

We fired him because he was kind of crazy. He promised to come back and kill us then,” she said.

Bill de Blasio, mayor of New York, described the attack as a “horrific situation in the middle of a place that people associate with care and comfort”.

He said it was not terrorism-related.

“We’ve had a real tragedy here in the Bronx this afternoon. It’s something we’ve seen around the country, and now we’ve experienced it here.”

The doctor killed in the attack was a lady, according to de Blasio, who said that several of the injured are “fighting for their lives”.

Bello was a graduate of Ross University School of Medicine on the Caribbean island of Dominica.

According to the New York Times, he was arrested in August 2004 after a 23-year-old lady accused him of s*xual harassment.

He was charged with s*x abuse and unlawful imprisonment. Dr Bello was said to have pleaded guilty to unlawful imprisonment in the second degree and was sentenced to community service.

The s*xual abuse charge was reportedly dismissed but he lost his job.