Police in the US city of Lewiston, Maine have warned a gunman is on the loose, with reports that at least 16 people have been killed.

Residents of the city, the second-largest in the state, have been told to shelter in place.

Police said they were investigating incidents at "multiple locations" and released photos of a suspect.

The White House said President Joe Biden had been briefed on the situation and would continue to receive updates.

There were also multiple but unconfirmed reports of at least 50 people injured.

In a statement on X, formerly Twitter, Maine State Police said: "There is an active shooter in Lewiston.

"We ask people to shelter in place. Please stay inside your home with the doors locked."

Lewiston Police said it had responded to two locations, a restaurant called Schemengees, and Sparetime Recreation, a bowling alley.

The two locations appear to be about four miles (6.5km), or 10-minute drive, from each other.

The Central Maine Medical Center said it was reacting to a "mass casualty, mass shooter event" and was coordinating with other hospitals in the area to treat the injured.

The Androscoggin County Sheriff's Office released two images of a suspect, saying he was at large and asking for the public's help identifying him.

They showed a bearded man in a brown sweater carrying a firearm walking into a building.

Police also shared a photo of a white vehicle, saying its front bumper was believed to be painted black, and asked anybody who recognised it to contact police.

Lewiston Public Schools Superintendent Jake Langlais said in a statement that schools in the district would be closed on Thursday.

A statement from the Department of Homeland Security (DHS) said Secretary Alejandro Mayorkas had been briefed and was continuing to monitor the situation.

"DHS is working closely with our federal, state and local partners to support the Lewiston community," it said.

A US justice department statement said that federal agencies were assisting state and local law enforcement.

The White House said President Biden had spoken individually by phone to Maine Governor Janet Mills, Senators Angus King and Susan Collins, and Congressman Jared Golden.

Senator King said in a statement he was "deeply sad for the city of Lewiston and all those worried about their family, friends and neighbors".

Lewiston is a town of about 38,000 people located about 35 miles north of Portland, Maine.