In the latest flare-up of tensions in the region, China’s military announced the resumption of military drills around the island in response to the unexpected arrival in Taiwan of a new delegation of US lawmakers.

According to Taiwan’s presidential office, the delegation, led by Senate Foreign Relations Committee member Ed Markey (D., Mass.), met with Taiwanese President Tsai Ing-wen on Monday to discuss regional stability and deepening US-Taiwan trade. The group arrived in Taipei on Sunday without having announced the two-day visit beforehand.

“We have a moral obligation to do everything in our power to avoid an unnecessary conflict,” Mr. Markey told Ms. Tsai, according to video footage of the meeting later shared by the presidential office.

In response to the visit, the Chinese military’s Eastern Theater Command’s spokesman, Maj. Shi Yi, said the unit was organizing joint war patrols and combat drills in the sea and airspace around Taiwan.

“This is a strong deterrent to the United States and Taiwan to continue playing political games and undermining peace and stability across the Taiwan Strait,” Maj. Shi said.

In a statement issued Monday evening, Taiwan’s Defense Ministry condemned China’s latest military exercises as “undermining regional peace and stability.” As of 5 p.m., China’s military, the People’s Liberation Army, had sent 30 aircraft on sorties near Taiwan, 15 of which crossed the median line, a notional line that bisects the Taiwan Strait separating Taiwan’s main island from the Chinese mainland.

Mr. Markey’s delegation’s arrival and China’s resumption of drills come roughly two weeks after House Speaker Nancy Pelosi’s visit to Taiwan, which prompted China to launch missiles over the democratically self-ruled island and encircle it with multiday live-fire military drills simulating a blockade. China’s military announced an end to those drills on Wednesday but said it would continue to conduct regular patrols of the Taiwan Strait.

Beijing claims Taiwan as part of its territory and has vowed to seize control of it, using force if necessary—a pledge reaffirmed in a new policy paper released last week, the country’s first in 22 years. China’s leadership condemned Mrs. Pelosi’s visit to Taiwan, the first by a House speaker in 25 years, as an attempt by Washington to upset the status quo in the Taiwan Strait.

According to people familiar with Chinese thinking, Beijing’s leaders are concerned that Mrs. Pelosi’s visit will spark a “domino effect” of other world politicians visiting Taipei, boosting its international standing and potentially encouraging it to formally declare independence.

Ms. Tsai thanked Mr. Markey’s delegation for coming “at such a critical time” during her meeting with them on Monday.

“As global authoritarianism grows, Taiwan hopes to deepen economic and trade cooperation with international friends who share democratic values, and jointly build a safer supply chain,” she said, referring to the island’s critical role in semiconductor supply.

According to the video released by Taiwan’s presidential office, Ms. Tsai and Mr. Markey discussed Taiwan’s semiconductor dominance, with Ms. Tsai saying she expected President Biden’s recent signing of the Chips Act to provide more incentives for Taiwanese investment in the US.

Chinese Foreign Ministry spokesman Wang Wenbin accused the U.S. of “hollowing out and distorting the one-China principle” during a regular news briefing in Beijing on Monday.

The “one-China principle” is Beijing’s position that the People’s Republic of China is the only sovereign and legitimate China, and that Taiwan is a part of it. In contrast, the United States maintains a “one-China policy,” which grants Beijing diplomatic recognition and acknowledges but does not support China’s claims to Taiwan.

For decades, the United States has avoided saying whether it would intervene in a conflict between China and Taiwan, a policy known as “strategic ambiguity.”

Wang Ting-yu, a member of Taiwan’s ruling Democratic Progressive Party who serves on the Foreign and National Defense Committee of the Taiwanese legislature, said that in a closed-door meeting on Monday, Taiwanese lawmakers and the American congressional delegation discussed whether Washington should shift from strategic ambiguity to “strategic clarity” on Taiwan.