New Hampshire state police initially had reported "multiple victims" from the shooting, but later said gunfire was confined to the lobby area of the facility and that "all patients are safe".

"The suspect in this situation is deceased," Colonel Mark Hall said, without elaborating or taking questions.

He also said there was no longer any threat to the public.

The Associated Press reports that, other than the suspect, one person was shot, citing an anonymous police official.

The shooting was "contained to the front lobby" of the hospital, according to the Homeland Security department, and authorities are investigating a "suspicious vehicle".

New Hampshire Governor Chris Sununu said the incident has been contained.

"While the scene remains active as the campus is cleared, the suspect is deceased," he said.

"The state immediately mobilised, and first responders and law enforcement are on the scene."

New Hampshire Hospital, which has roughly 185 beds, is the state psychiatric hospital, located in the state's capital city, Concord.

An armoured vehicle was seen approaching the scene outside the hospital.

Read more:
America's most deadly mass shootings of 2023
Mass shootings are elsewhere - but America's problem with guns stands alone

Aerial views of the hospital earlier showed an active scene with numerous police cars outside the hospital.

The shooting is the latest act of violence at a hospital in the US, with medical centres across the country struggling to adapt to growing threats.