A Japanese commercial ship and a US nuclear-powered submarine collided in the Arabian Sea on Tuesday, media reports said.
No injuries or a major oil leak has been detected, the reports said, quoting government and company officials.
The Japanese Foreign Ministry received the information about the collision at about 4 am (1900 GMT), but details are not yet known, Kyodo News Agency said.
The Japanese ship involved was the 16-tonne tanker Mogamigawa, owned by Kawasaki Kisen Kaisha Ltd, with eight crew members aboard and currently based out of the UAE, Jiji Press quoted a government official as saying.
The US Navy submarine was based in Bahrain.
The collision was the second in six years between a Japanese vessel and a US submarine. In February 2001, a submarine rammed the fishing trawler Ehime Maru during a surfacing manoeuvre off the Hawaiian islands.