Four passengers were killed and 33 others injured when a Tokyo subway train derailed and sideswiped a packed commuter train running in the opposite direction during rush hour Wednesday morning, police said.

The collision occurred just after 9 a.m. about 100 meters northeast of Nakameguro Station, in Tokyo's Meguro Ward, on the Hibiya Line operated by the Teito Rapid Transit Authority subway company, police said.

This is the first major accident Teito has seen in nearly 70 years of operation and the first time passengers' lives were lost.

Investigators have not determined the exact cause of the derailment, they said.

Police identified the victims as Shinsuke Tomihisa, 17, an Azabu High School student from Yokohama's Nishi Ward; Tomomi Yamazaki, 29, a businesswoman from Yokohama; Yasuyo Maki, 37, a female employee of Kagoshima-based newspaper Minami Nihon Shimbun; and Shinya Fujii, 33, official of Patent Office.

Maki was rushed to a hospital but died in the afternoon, and Fujii was confirmed dead late in the evening.

Among the injured, four remain in critical condition and another six are severely injured, according to police.

The injured passengers were being treated at the Japanese Red Cross Medical Center, Hiroo Hospital, Kosei Chuo Hospital, Showa University Hospital, Mishuku Hospital, Tokyo Women's Medical College Hospital, Keio University Hospital and Tokyo Medical College Hospital.

Chief Cabinet Secretary Mikio Aoki said the accident was not caused by an explosion or any act of terrorism, although he added the government has yet to be informed of why the train derailed.

The collision occurred as the eight-car Teito train was about to arrive at Nakameguro Station, the terminus of the Hibiya Line.

Just after emerging from the tunnel, the lead wheels of the rear carriage derailed and it collided with a northbound eight-car train running on a parallel track, police said.

A portion of the derailed car was sheared off like an opened tin can.

Experts said since there are less than 80 cm between trains on adjacent tracks, a crash is inevitable once one train derails.

The other train was operated by Tobu Railway Co., which runs on Teito tracks at the site.

The Teito train was heading from Kitasenju to Kikuna with about 240 passengers, while the Tobu train, leaving Nakameguro for Takenotsuka, was packed with about 1,300 passengers, Teito officials said.

The four who died were all aboard the Tobu train, officials of the Tokyo Metropolitan Fire Department said.

Tokyu and Teito share tracks between Nakameguro and Kikuna, and the two lines meet at Nakameguro Station.

Railway officials said that no irregularities were reported in the wheels of the derailed train when it went through a full maintenance test in September 1998.

The train was originally introduced in 1988 and has so far been operated for more than 1 million kilometers. The 28-year-old driver of the derailed train had nearly seven years of experience, the officials said.

They also said no irregularities were detected in the rails during a visual inspection Saturday and that worn rails were replaced in November.

Operations between Kasumigaseki and Ebisu on the Hibiya Line were resumed at 9:20 p.m., while the service between Ebisu and Nakameguro remained closed throughout Wednesday.

The Hibiya Line is expected to resume full service once tracks at the derailment site are replaced and inspected, the officials said.