Three people, including two police officers, were killed on Thursday in a shooting and stabbing attack in the central region of Nagano, Japanese media said.
A male suspect was holed up inside a building after the incident, according to public broadcaster NHK and other major outlets.
Police did not immediately confirm details when contacted by AFP.
Residents were urged to stay indoors by authorities in the area around Nakano, where the attack took place.
A woman was stabbed and two police officers were shot in the afternoon attack at a farm, the reports said.
An eyewitness told NHK he had been working nearby when he saw a woman being stabbed.
"A woman came running from the road saying 'help me' and she fell down," the man said.
"Behind her came a man wearing camouflage and carrying a large knife, who stabbed her in the back."
The witness said he had called the police while neighbours tried to resuscitate the woman.
NHK, citing police, said the attacker later allegedly fired what appeared to be a shotgun at the police officers who arrived on the scene.
Violent crime is rare in Japan, which has a low murder rate and some of the world's toughest gun laws.
But the country was left reeling in July when former prime minister Shinzo Abe was assassinated in broad daylight with an apparently homemade gun.
And last month, a man was arrested for allegedly hurling a pipe bomb-like explosive toward current Prime Minister Fumio Kishida as he campaigned in the western city of Wakayama.