TOKYO (Reuters) - Japan's ambassador-designate to China, Shinichi Nishimiya, died on Sunday in a Tokyo hospital, the Foreign Ministry said, three days after he was found unconscious on a Tokyo street.

Doctors were looking into the cause of death, ministry official Takashi Ariyoshi said in a statement, but no other details were available. Nishimiya, 60, was found unconscious on a street near his home on his way to work.

Nishimiya was to have left in mid-October to take over from Uichiro Niwa as Japan's top envoy in Beijing.

While coincidental, Nishimiya's death came as tensions flared up between Japan and China over a disputed group of islands in the East China Sea claimed by both countries.

Anti-Japan protests flared in cities across China for a second day on Sunday, with police using tear gas and water cannon to drive back thousands of protesters in the southern Chinese city of Shenzhen, near Hong Kong.

The protests, the latest setback in long-troubled relations between China and Japan, followed Japan's decision on Tuesday to buy the disputed islands, called Senkaku by Tokyo and the Diaoyu by Beijing and which could contain rich gas reserves, from a private Japanese owner.

(Reporting by Yuko Inoue; Editing by Paul Tait)

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