Following a week of diplomatic barb-trading, during which Beijing said it was “fully prepared for any eventuality” and Washington continued to downplay the potential fallout of the as-yet-unconfirmed trip, China’s defense ministry weighed in on the topic for the first time.

Tan Kefei, the ministry’s spokesperson, said a visit to Taiwan by “the U.S. government’s No. 3 figure” would violate China’s sovereignty and territorial integrity, and “seriously undermine the political foundation of China-U.S. relations.”

The speaker’s trip, which the Financial Times first reported would take place as part of a wider visit to Asia next month, would “inevitably cause extremely serious harm to state-to-state and military-to-military relations between China and the United States, leading to further tension in the Taiwan Strait,” he said.

When the U.S. and China established formal diplomatic relations in 1979, Washington committed to a “one China” policy that recognized the legitimacy of the government in Beijing, and neither accepted nor rejected Chinese claims of sovereignty over Taiwan, the self-governing democratic island that over the decades has shown a declining interest in a political union with the neighboring mainland.

The U.S. considers Taiwan’s postwar political status to be undetermined. Longstanding American legislation—supported by Joe Biden when he was in the Senate—requires Washington to supply Taipei with defensive arms to withstand military coercion, in order for differences across the Taiwan Strait to be resolve peacefully.

China considers this continued support as interference in its internal affairs, despite the U.S.’s official policy of not endorsing Taiwanese independence. Beijing also believes the White House should exercise executive powers to block Pelosi’s visit to Taipei.

“China demands that the U.S. side take practical action to fulfill its commitment of not supporting Taiwan independence, and not to facilitate Pelosi’s visit to Taiwan,” said Tan. “If the U.S. side insists on going its own way, the Chinese military will not sit back, and will take forceful measures to thwart any interference by external forces and secessionist attempts at Taiwan independence.”

Pelosi’s office is yet to confirm her travel plans to Taiwan or the rest of the region, citing security protocols. The California Democrat, who will reportedly travel with a bipartisan congressional delegation, would be the first sitting speaker of the House of Representatives to visit Taipei in a quarter-century.

Last week, President Biden all but confirmed the speaker’s plans when he hinted at the Defense Department’s opposition to them, describing the Pentagon’s view of the trip as “not a good idea right now.”

Pelosi brushed off the comments the next day, telling reporters on Thursday: “I think what the President was saying is maybe the military was afraid our plane would get shot down or something like that by the Chinese. I don’t know exactly.”

“I think it is important for us to show support for Taiwan,” she said. “I also think that none of us has said they are for independence when it comes to Taiwan. That’s up to Taiwan to decide.”

During her time in Congress, Pelosi has been known to take a hard line on Beijing, a stance that has seen her support democratic freedoms in Hong Kong, criticize human rights abuses in Xinjiang and back the government in Taiwan. But the speaker’s plans could undermine a significant political milestone for China’s President Xi Jinping later this year.

Leaders of the Chinese Communist Party will gather at the Beidaihe beach resort to iron out their differences in August ahead of the party’s twice-a-decade national congress in October or November, during which Xi is expected to secure an unprecedented third term and cement his cult status within the party.

Part of this process requires Xi to legitimize his continued rule by demonstrating leadership and resolve to safeguard Beijing’s core interests, chief among which is Taiwan. Pelosi’s plans, although not strictly endorsed by the Biden administration, may be perceived as a slight to the Chinese leader, necessitating a firm response.

Difficult to Back Out

But in the U.S., where domestic political dynamics are equally influential, Pelosi could find it increasingly difficult to back out of the trip without appearing to have bowed to Chinese pressure, at a time when both Democrats and Republicans are urging her to go.

“What is the Pentagon thinking when it publicly warns against Speaker Pelosi going to Taiwan? If we are so intimidated by the Chinese Communists we can’t even protect an American Speaker of the House why should Beijing believe we can help Taiwan survive. Timidity is dangerous,” read a tweet on Monday by the GOP’s Newt Gingrich, the last serving House speaker to visit Taiwan in 1997.

Rep. Ro Khanna, a California Democrat who sits on the House Armed Services Committee, told CNN that Pelosi shouldn’t call off her visit. “We’re not going to let the Chinese Communist Party dictate where the speaker of the House should go.”

On Tuesday, Senate Minority Leader Mitch McConnell described the stakes in clearer terms. “If she doesn’t go now, she’s handing China a sort of a victory of sorts,” he told a news conference of Senate Republican leaders.

Taipei, for its part, welcomes visits from Congress and is unlikely to decline Pelosi’s travel out of concerns that its already fraught relationship with Beijing will deteriorate further. For the time being, however, it’s remained quiet and chosen not to add to the cacophony of opinions.

Joanne Ou, Taiwan’s foreign ministry spokesperson, told Newsweek that Taipei hasn’t received details about Pelosi’s visit and would therefore not comment further.