A woman and two male police officers were killed in a shooting and stabbing attack at a farm in central Japan on Thursday, media reported.

A suspect, believed to be a farmer in his 30s, was holed up inside a building after the incident, which also left a fourth person injured, the reports said.

Residents were urged by authorities to stay indoors in the semi-rural area around the city of Nakano in Nagano region, where the attack took place.

It was a rare instance of violent crime in Japan, which has a low murder rate and some of the world's toughest gun laws.

Public broadcaster NHK and other major outlets said the woman was stabbed and the two police officers were shot in the afternoon attack.

A witness told NHK he had been working at a farm nearby when "a woman came running from the road saying 'help me', and she fell down".

"Behind her came a man wearing camouflage and carrying a large knife, who stabbed her in the back," the 72-year-old witness said.

He said he called emergency services while neighbours tried to resuscitate the woman.

NHK, citing police, said the attacker then fired what appeared to be a shotgun at the police officers who arrived at the scene.

The officers were inside a patrol car and the attacker placed the weapon against a window of the vehicle and fired twice, NHK reported.

Another man was injured in the attack but could not be rescued because he was too close to the building where the suspect has taken shelter, Kyodo News said. (AFP)