Beijing’s defense ministry has warned that the Chinese military would “never sit idly by” and take “strong measures” if U.S. House Speaker Nancy Pelosi makes a rumored visit to Taiwan.

The comments by ministry spokesman Colonel Tan Kefei, made during a press briefing on Tuesday, amounted to China’s starkest threat yet over a contentious potential trip that has set relations between Washington and Beijing on edge.

“If the U.S. insists on going its own way, the Chinese military will never sit idly by, and will take strong measures to thwart any external interference,” Tan said.

“China requires the U.S. to take concrete actions ... not to arrange for Pelosi to visit Taiwan.”

The California Democrat is planning a trip to the self-governing island next month, according to a report in the Financial Times last week. The possibility of a visit prompted a series of angry responses from Beijing’s foreign ministry, but it wasn’t until Tuesday that the military directly addressed the scenario.

Pelosi has not confirmed the trip, which would be the first to Taiwan by a House speaker since Republican Newt Gingrich met then-President Lee Teng-hui in 1997. She had reportedly planned to visit in April during a scheduled Asia tour, but the trip was canceled after she tested positive for COVID-19.

U.S. President Joe Biden weighed in on the issue last week, saying that the U.S. military believes “it’s not a good idea right now” for Pelosi to make the visit.

Biden and Chinese President Xi Jinping are scheduled for a phone call on Thursday amid the worsening ties, Bloomberg reported.

A number of prominent Republicans have encouraged the Pelosi trip.

“Speaker Pelosi should go to Taiwan and President Biden should make it abundantly clear to Chairman Xi that there’s not a damn thing the Chinese Communist Party can do about it,” Sen. Ben Sasse, R-Neb., said on Monday. “No more feebleness and self-deterrence.”

Senate Republican leader Mitch McConnell said during a news conference Tuesday that Pelosi would hand China “a victory of sorts” if she backed off on the visit after threats from Beijing.

China views the democratic island of 23 million as a wayward province and has vowed to retake it by force if necessary. Concerns over Beijing’s plans for Taiwan have grown in the wake of the Russian invasion of Ukraine, which is now in its sixth month.

China has also ratcheted up military provocations against Taiwan over the past several months, with frequent incursions into the island’s Air Defense Identification Zone. Beijing also recently amplified sovereignty claims over the Taiwan Strait.

The United States has responded by staging joint naval exercises with allies in the region and sailing warships during freedom of navigation exercises. Last week, the guided missile destroyer USS Benfold transited the Taiwan Strait on its third crossing in a week of China-claimed international waters.

Taiwan also kicked off annual weeklong military exercises on Monday, which included air raid drills in the capital Taipei focused on a potential invasion by China.