Japanese police on Friday detained a suspect who had holed up in a building after allegedly killing four people, including two police officers, in a gun and knife attack.

The man was taken into custody outside a farm near the city of Nakano in Nagano region, with police confirming a fourth fatality overnight – an elderly woman who was found injured at the scene and later pronounced dead.

Another woman and two police officers were earlier confirmed dead in the attack.

Police said that the 31-year-old suspect had been detained at about 4:30 am (1930 GMT Thursday), and later confirmed he had been formally arrested on suspicion of murder using a suspected hunting rifle.

The rampage in a rural area of the western region was a rare instance of violent crime in Japan, which has a low murder rate and some of the world's toughest gun laws.

No motive has yet emerged in the killings, nor has the suspect been formally identified, though several local media reported he is the son of the speaker of Nakano's city assembly. (AFP)