China said Friday that it has started military drills in the Taiwan Strait and surrounding waters, amid tensions over the recent visit by a senior U.S. State Department official to the self-governed island.

"This is a legitimate and necessary action taken in response to the current situation across the Taiwan Strait to safeguard national sovereignty and territorial integrity," a spokesman for the People's Liberation Army told reporters in Beijing.

Shiyu, or Lion Islet, part of Kinmen County, one of Taiwan's offshore islands, is seen in front of the Chinese city of Xiamen, China, on April 20, 2018 in Kinmen, Taiwan. (Getty/Kyodo)  

His remarks came a day after Keith Krach, U.S. undersecretary of state for economic growth, energy and the environment, arrived in Taiwan to attend the memorial service for former President Lee Teng-hui.

Washington has said his trip was aimed at honoring Lee's legacy "by continuing our strong bonds with Taiwan." Krach met Friday with independence-leaning Tsai Ing-wen, who has served as Taiwan's president since 2016.

Tsai expressed gratitude for Krach's visit and said that it conveyed the U.S. government's support for democratic Taiwan. She also noted that Taiwan-U.S. relations have made substantial progress in recent years, adding that she hopes the two sides will have closer exchanges as well as joint efforts in more areas to advance further cooperation, according to a release from Taiwan's presidential office.

Taiwanese President Tsai Ing-wen delivers an inauguration speech after taking the oath of office for her second term on May 20, 2020. (Central News Agency/Kyodo)

Also Friday, Taiwan's air force scrambled planes to warn off 18 Chinese warplanes crossing the median line of the Taiwan Strait and flying over waters southwest of Taiwan's air defense identification zone, the island's Defense Ministry said.

The movement was observed based on a real-time platform the ministry established on its website. The planes were identified as eight J-16 fighters, four J-11 and four J-10 fighters and two H-6 jet bombers.

Emphasizing that the Taiwan issue is an internal affair, the spokesman said the United States and the island's ruling party "have stepped up their collusion, frequently creating disturbances."

Taiwan and mainland China have been separately governed since they split amid a civil war in 1949. Their relationship has deteriorated under the Tsai government.

Washington switched its diplomatic recognition from Taipei to Beijing in 1979, but the administration of U.S. President Donald Trump has been trying to boost ties with the democratic island.

Last month, Health and Human Services Secretary Alex Azar became the most senior U.S. official to visit Taiwan since 1979, inflaming the already strained tensions between the world's two major powers.

Following his visit, China also held military drills involving several units in and around the Taiwan Strait.

In Taiwan, fears linger that the communist-led Chinese government, headed by President Xi Jinping, will attempt to achieve its cherished goal of reuniting the island with the mainland, by force if needed, foreign affairs experts say.

Lee, Taiwan's first popularly elected leader, died at age 97 in late July.