The self-governing island on Monday kicked off its Han Kuang military exercises, an annual five-day event simulating an invasion by China. While Taiwan’s drills were previously planned, Pelosi’s possible visit comes amid heightened tensions with Beijing and has drawn strong reactions in both the U.S. and China.

Taiwan’s Ministry of National Defense ordered F-16V jets from Hualien Air Force Base to conduct emergency takeoffs to simulate a scenario where they’d have to defend the country’s east coast, reports the Taipei Times. Additionally, the country’s Navy and ground troops carried out readiness drills designed to resemble attacks by enemy planes or missiles.

Television station Focus Taiwan also captured video of air raid sirens blaring through mostly vacant city streets.

Reports emerged last week that Pelosi was planning to bring a congressional delegation to Taiwan, a democratically governed island that has long had the economic and military backing of the U.S. A visit by the California Democrat would be the first to the island by a serving House speaker in 25 years. Republican Newt Gingrich visited in 1997.

Chinese officials have pledged to reclaim Taiwan, which they say is a breakaway province. U.S. commitments to the island have come into focus amid saber-rattling in Beijing and Russia’s invasion of Ukraine.

But Pelosi’s possible visit has not been embraced by the Biden administration, despite its stated aim of seeking to reduce China’s influence.

“Well, I think that the military thinks it’s not a good idea right now,” President Joe Biden told reporters last week when asked about Pelosi’s trip. “But I don’t know what the status of it is.”

General Mark Milley, chairman of the Joint Chiefs of Staff, told Associated Press in an interview published Sunday that the U.S. and its allies have seen more intercepts by Chinese aircraft and ships in the Pacific region, with a rise in unsafe interactions.

“The message is the Chinese military, in the air and at sea, have become significantly more and noticeably more aggressive in this particular region,” said Milley.

Pelosi’s visit has also drawn a stern reaction from Chinese officials, who’ve raised concern about what the trip would mean for the country’s sovereignty and territorial integrity.

Chinese Foreign Ministry spokesman Zhao Lijian reiterated those worries at a press conference Monday, according to Xinhua news agency.

“The Chinese side has repeatedly made clear to the U.S. side our serious concern over Pelosi’s potential visit to Taiwan and our firm opposition to the visit,” he said, adding, “We are fully prepared for any eventuality.”

Meanwhile, Republican members of Congress have called on Pelosi to push ahead with the visit, saying that backing down would be a show of weakness. GOP Representative Tom Tiffany of Wisconsin and Congressman Scott Perry of Pennsylvania wrote in a letter to Pelosi that she should “reject the objections of China’s rulers and President Biden,” reported Focus Taiwan.

GOP Representative Mike Gallagher, also of Wisconsin, said Monday on a Fox News appearance that China is already trying to make the U.S. lose its will to defend Taiwan. He said federal law calls for increased interactions between leaders of Taiwan and the U.S.

“If we’re too afraid to let that small thing happen, how are we ever going to expedite arms sales to Taiwan?” Gallagher said.

A spokesperson for Pelosi told Newsweek in an email, “We do not confirm or deny international travel in advance due to longstanding security protocols.”

Correction 07/26/22, 1:25 p.m. ET: Corrects spelling of name of Chinese Foreign Ministry spokesman