BEIJING (AP) — China will take “firm and strong measures” if U.S. House of Representatives Speaker Nancy Pelosi plans to visit Taiwan, China’s foreign ministry said Tuesday.

According to the Financial Times, Pelosi, who is second in line for the presidency, is scheduled to visit the self-governing island claimed by China in August.

She was originally scheduled to visit in April but postponed after she tested positive for COVID-19.

Pelosi will be the highest-ranking US lawmaker to visit America’s closest ally since her predecessor as speaker, Newt Gingrich, traveled there 25 years ago.

China has vowed to annex Taiwan by force if necessary and has advertised that threat by flying warplanes near Taiwan’s airspace and conducting military exercises based on an invasion scenario. It said the actions were aimed at preventing advocates of the island’s formal independence and foreign allies – mainly the US – from coming to its aid, 70 years after the sides split in a civil war.

Pelosi’s visit “will seriously undermine China’s sovereignty and territorial integrity, seriously affect the foundation of Sino-US relations, and send a seriously wrong signal to Taiwan independence forces,” Foreign Ministry spokesman Zhao Lijiang said at a daily briefing.

“If the US insists on going down the wrong path, China will take firm and strong measures to protect its sovereignty and territorial integrity,” Zhao said.

China has stepped up its rhetoric in recent days over US arms sales to Taiwan, calling for the cancellation of an estimated $108 million deal that would increase the survivability of its armed forces against its archenemy. China has the world’s largest standing army in the 180-kilometer (100-mile)-wide Taiwan Strait, with an increasingly sophisticated navy and a huge inventory of missiles.

“The Chinese People’s Liberation Army … will resolutely rebuff any interference from outside forces and separatist machinations for ‘Taiwan independence,'” the defense ministry said in a statement posted on its website on Tuesday.

While Washington maintains a policy of “strategic ambiguity” over whether it will defend Taiwan in a conflict with China, US law requires it to ensure the island has the means to defend itself and threats to its security are a matter of “serious concern”.

Washington maintains only informal relations with Taiwan in relation to Beijing, but is the island’s strongest political ally and source of defensive weapons.

Zhao did not provide any details on what possible actions China might take in response to Pelosi’s visit, but Beijing has commonly used military airstrikes and war games to show its displeasure. Chinese pilots have also been accused of aggressively targeting surveillance aircraft from the US and its allies operating in international airspace off China’s coast, using lasers and other methods to harass foreign warships in the South China Sea.

China’s most serious threat against Taiwan came in 1995-96, when it conducted military exercises and fired missiles into waters north and south of the island in response to a visit to the US by then-President Lee Teng-hui.

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