Eight Chinese war planes crossed the Taiwan Strait’s median line and approached close to the island’s contiguous zone that is 24 nautical miles (44 km) off its coast on Saturday, the Taiwanese defense ministry said, in a further rise in tensions.

China’s air force over the past three years has routinely flown into the air space around Taiwan. Since August it has regularly sent jets across the strait’s median line, which had previously served as an unofficial barrier.

Beijing claims democratically governed Taiwan as China’s territory. Taiwan’s government rejects the claims and says only the island’s people can decide their future.

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In March, Taiwan Defense Minister Chiu Kuo-cheng warned that the island had to be on alert this year for a “sudden entry” by the Chinese military into areas close to its territory.

Taiwan defines its contiguous zone as being 24 nautical miles from its coast. Its territorial space is defined at 12 nautical miles from its coast, though the government has not reported Chinese aircraft entering either the contiguous zone or Taiwan’s territorial air space.

Taiwan’s defense ministry said in a statement that starting around 8 a.m. (0000 GMT) on Saturday, it had detected 19 Chinese warplanes, including J-10 and J-16 fighters. Eight of those crossed the median line and “got close” to the 24 nautical mile mark, it said.

At the same time, the ministry said five Chinese warships carried out what it termed “joint combat readiness patrols.”

Taiwan sent up its own aircraft and dispatched ships in response and activated land-based missile systems, the ministry said, using standard wording for how its forces react to Chinese incursions.

China’s defense ministry did not answer calls seeking comment.

On Saturday, the Chinese military’s Eastern Theater Command, which covers Taiwan, published pictures on its WeChat account of J-16 fighters taking off on a training mission “far out at sea” that happened “recently.”

The command said the aircraft carried out “tactical long-range flight training” involving in-flight refuelling that covered “thousands of kilometers.” It did not give details.

During China’s last large-scale war games near Taiwan, in April, videos of the exercise released by the Eastern Theater Command showed its vessels and planes coming within 24 nautical miles of Taiwan on all sides of the island.

The Taiwan Strait, which separates the island from its giant neighbour China, is also a frequent site of US military activities, especially US warships which transit the sensitive waterway about once a month.

The US Navy this month released a video of what it called an “unsafe interaction” in the strait, in which a Chinese warship crossed in front of a US destroyer.

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