TOKYO, Sept 16 (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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  • Senkaku, or Diaoyu, Islands

    Located in the East China Sea near Taiwan and the southern Japanese island of Okinawa, these remote uninhabited isles have been under Japanese control since 1895. They are seen as important because of their strategic location, and are surrounded by rich fishing grounds and may be near underwater resources such as natural gas. China claims it discovered them in the 14th century. Claimed by Japan, China and Taiwan. <br><em>An anti-Japan protester shouts slogans near a Chinese national flag outside the Japanese Embassy in Beijing, China, on Friday, Aug. 17, 2012. (AP Photo/Alexander F. Yuan)</em>

  • Dokdo, or Takeshima

    Administered by South Korea since the 1950s, these outcroppings in the Sea of Japan, called the East Sea in Korea, are inhabited only by a contingent of South Korean police. Claimed by South Korea and Japan. <br><em>South Korean protesters shout slogans during a rally against Japan's sovereignty claims over the islet of Dokdo in South Korea, which is known as Takeshima in Japan, in front of the Japanese Embassy in Seoul, South Korea, on Friday, Aug. 17, 2012. (AP Photo/Ahn Young-joon)</em>

  • Kuril Islands

    Located off the Russian Far East and Japan's northernmost main island, the southern Kurils were occupied by the Soviets in the closing days of World War II, and Japan before that. Four of the Russian-controlled islands, which have small military and civilian populations, are in dispute and have kept Japan and Russia from signing a formal treaty ending their wartime hostilities. They are a base for fishing operations and a rich source of crab. Claimed by Russia and Japan. <br><em>Japanese Prime Minister Shinzo Abe speaks in front of a map of islands, known as Northern Territories in Japan and Kuril islands on February 7, 2007, in Tokyo, Japan. Held in Russia, the meeting demanded the return of the islands. (Photo by Junko Kimura/Getty Images)</em>

  • Spratly Islands

    A flashpoint in the South China Sea, they are comprised of hundreds of coral reefs, islets and atolls claimed entirely or in part by China, Taiwan, Vietnam, Malaysia, Brunei and the Philippines. <br><em>This July 20, 2011, file photo shows an aerial view of the Pag-asa Island, part of the disputed Spratly group of islands, in the South China Sea located off the coast of western Philippines. (AP Photo/Rolex Dela Pena, Pool, File)</em>

  • Paracel Islands

    About halfway between China and Vietnam in the South China Sea, they are claimed by China, Vietnam and Taiwan. They are called Xisha in Chinese and Hoang Sa in Vietnamese. China and Vietnam had a conflict over them in the 1970s, and China has controlled them since then. <br><em>This July 27, 2012, photo shows an aerial view of Sansha, a city on the disputed Paracel islands, which is now considered by China as a part of the Hainan province. (STR/AFP/GettyImages)</em>