Russia will not prosecute the crew of a border patrol boat over a clash in which they killed a Japanese fisherman because they acted in self-defence, local media quoted the chief military prosecutor as saying on Monday, according to Reuters. The death was the first in 50 years in the dispute between Russia and Japan over ownership of four sparsely populated islands off northern Japan and provoked diplomatic protests from both sides. Russian Foreign Minister Sergei Lavrov complained to his Japanese counterpart on Monday about what he said were increasingly frequent border violations by Japanese vessels, his ministry said in a statement. "The Japanese tried to sink the border guards' vessel by ramming it," Interfax news agency quoted Chief Military Prosecutor Sergei Fridinsky as saying. "It was precisely to prevent a collision that the (border patrol's) captain opened fire, as a result of which a Japanese seaman died." The clash happened on Aug. 16 in disputed waters east of Hokkaido between Japan and Russia. Moscow said the Japanese crab vessel was fishing illegally in Russian territorial waters. "Our investigation has established that our border guards acted in accordance with the requirements of Russian law, and that weapons were used with justification," Fridinsky said in comments broadcast by Channel One television. "For these reasons today we have decided not to press criminal charges against our border guards because their actions did not constitute a crime," said Fridinksy. Lavrov, in a telephone conversation with Japanese Foreign Minister Taro Aso, said Japanese vessels were violating Russian borders "in spite of repeated appeals by the Russian side to the Japanese authorities," the statement said. "(Lavrov) expressed the hope that the Japanese side will carry out its promises and take effective measures to stop poaching in waters under Russia's jurisdiction." Both nations claim sovereignty over the Russian-controlled islands, known as the Northern Territories in Japan and the Southern Kuriles in Russia. The dispute has prevented the two countries signing a peace treaty more than 60 years after the end of World War Two. The last Japanese death in the dispute was in October 1956, when a Japanese fisherman was shot dead by a Soviet vessel, Japanese officials said.