At least 40 people have been injured in the knife attack in the city of Sagamihara.

A knifeman who killed at least 19 at a care facility near Tokyo, Japan, was reportedly an ex-staff member who wanted to "get rid of the disabled from this world".

Police responded to a call about 2.30am from an employee at the Tsukui Yamayuri En (Tsukui Lily Garden) care facility. The worker told them something horrible was happening.

A 26-year-old was beginning his rampage.

AP Ambulance crew and police officers at the scene of the attack.

Local media named him as Satoshi Uematsu, though police have yet to confirm that.

The number of people he allegedly injured at the facility in the city of Sagamihara has not been confirmed. Reports put the number between 15 and 45.

Some time after the attack began, a man turned himself in at a police station, officers said. He left the knife in his car when he entered the station.

AP The assailant had reportedly been employed at the care facility.

He has been arrested on suspicion of attempted murder and trespassing.

"I did it," he reportedly told officers.

The attack is possibly one of Japan's worst single-perpetrator mass murders in the postwar era.

AP Ambulance crew and police officers are seen outside the facility where the massacre happened.

News outlet Asahi Shimbun said police quoted him as saying: "I want to get rid of the disabled from this world."

Police would not confirm the number of dead, injured, nor the arrested man's name, the New York Times reported.

But media in Japan said Uematsu, a former employee of the facility, broke in and began screaming. "All the handicapped should disappear," he reportedly yelled as the bloody attack began.

KYODO Police officers outside the facility where at least 19 people were killed, in Sagamihara, Japan.

The facility lies 48km outside Tokyo, the world's safest city.

The city of Sagamihara last made international news in 2012, the New York Times reported, when one of the suspects in the 1995 poison gas attack on the Tokyo subway system was arrested there.

Naoko Kikuchi, one of the most wanted people in Japan for her involvement in the attacks that killed 13 people and injured thousands, had been hiding in the town under the name Chizuko Sakurai.