Five people were unaccounted for Wednesday after a South Korea-flagged chemicals tanker with 11 people on board capsized in rough seas off Japan, the coastguard said.

Six members of the crew -- which included two South Koreans, eight Indonesians and one Chinese -- had been rescued so far, and the coastguard is "still searching for the remaining five," a spokesman told AFP.

The tanker was "carrying acrylic acid, but there is no information yet on whether that has leaked into the ocean," he added, without wishing to be named.

He said he was not aware of the conditions of those rescued, who were rushed to hospital.

Footage from Japanese broadcaster NHK showed the overturned red hull of the ship as well as a life raft, as a coastguard ship pounded through heavy waves and a helicopter flew overhead.

The crew notified the coastguard early on Wednesday that the vessel was tilting and requested help near the island of Mutsure, off Japan's southwestern coast, NHK said.

The Japan Coast Guard received a rescue call soon after 7:00 am (2200 GMT Tuesday) saying that the ship it was "tilting, please help us", the spokesman said.

NHK named the vessel as the Keoyoung Sun, which specialist website vesselfinder.com said is a chemical and oil products tanker built in 1996, measuring 69 metres (226 feet) in length.

The ship's operator declined to comment.