Japan's Defense Ministry says a Chinese government helicopter briefly entered Japanese airspace near the Senkaku Islands in Okinawa Prefecture on Saturday afternoon.



Japan controls the islands. China and Taiwan claim them.



Ministry officials say Japan Coast Guard patrollers first detected an official Chinese vessel entering Japanese waters about 22 kilometers off the islands.



They say a helicopter then took off from the Chinese ship and flew within Japanese airspace for about 15 minutes. Officials say the Japan Air Self-Defense Force scrambled fighter jets, but did not use weapons or signal flares.



The officials believe the Chinese side was reacting to a small Japanese civilian aircraft flying near the islands moments earlier.



The Japanese government maintains the Senkaku Islands are an inherent part of Japan's territory, in terms of history and international law. It says there is no issue of sovereignty to be resolved over them.



Japan's Defense Ministry says it has lodged a strong protest with China through diplomatic channels and strongly urged Beijing to prevent a recurrence.



Vice Foreign Minister Funakoshi Takehiro lodged a similar protest with China's Ambassador to Japan, Wu Jianghao.



This is the fourth time Chinese aircraft have violated Japanese airspace. Three of the incursions took place off the Senkaku Islands, but this is the first time a Chinese government helicopter was involved.



The Defense Ministry and the Japan Coast Guard remain on the alert, as official Chinese vessels have been sailing just outside the islands' territorial waters almost on a daily basis.