Taiwan said Saturday that China’s military drills appear to simulate an attack on the self-ruled island, after multiple Chinese warships and aircraft crossed the median line of the Taiwan Strait.

Taiwan’s armed forces issued an alert, dispatched air and naval patrols around the island, and activated land-based missile systems in response to the situation, Taiwan’s Ministry of National Defense said on Twitter.

China launched live-fire military drills following U.S. House Speaker Nancy Pelosi’s trip to Taiwan earlier this week, saying that her visit violated the “one-China” policy. China sees the island as a breakaway province to be annexed by force if necessary, and considers visits to Taiwan by foreign officials as recognizing its sovereignty.

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Separately on Saturday, Taiwan’s army detected four unmanned aerial vehicles flying in the vicinity of the offshore county of Kinmen, Taiwan’s Central News Agency reported.

The four drones, which Taiwan believed were Chinese, were spotted over waters around the Kinmen island group and the nearby Lieyu Island and Beiding islet, according to Taiwan’s Kinmen Defense Command.

Taiwan’s military fired warning flares in response.

“Our government & military are closely monitoring China’s military exercises & information warfare operations, ready to respond as necessary,” Taiwan’s President Tsai Ing-wen said in a tweet.

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“I call on the international community to support democratic Taiwan & halt any escalation of the regional security situation,” she added.

Kinmen, also known as Quemoy, is a group of islands administered by Taiwan. The islands are located roughly 10 kilometers (6.2 miles) east of the Chinese coastal city of Xiamen in Fujian province.

The Chinese military exercises began Thursday and are expected to last until Sunday. So far, the drills have included missile strikes on targets in the seas north and south of the island in an echo of the last major Chinese military drills in 1995 and 1996 aimed at intimidating Taiwan’s leaders and voters.

Taiwan has put its military on alert and staged civil defense drills, while the U.S. has numerous naval assets in the area.

The Biden administration and Pelosi have said the U.S. remains committed to a “one-China” policy, which recognizes Beijing as the government of China but allows informal relations and defense ties with Taipei. The administration discouraged but did not prevent Pelosi from visiting.

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China has also cut off defense talks with the U.S. and imposed sanctions on Pelosi in retaliation for the visit.

Pelosi has been a long-time advocate of human rights in China. She, along with other lawmakers, visited Beijing’s Tiananmen Square in 1991 to support democracy two years after a bloody military crackdown on protesters at the square.