China has issued stern private warnings to the Biden administration over the upcoming trip to Taiwan by U.S. House of Representatives Speaker Nancy Pelosi, setting off alarm bells among White House officials who are worried. oppose his visit.

Six people familiar with the Chinese warnings said they were significantly stronger than threats Beijing has made in the past when it was unhappy with US actions or policy toward Taiwan.

China has publicly threatened “strong measures” if Pelosi goes ahead with the planned visit in August. But one person said China has expressed “stronger opposition” to the United States privately than before. Several other people familiar with the situation said the private rhetoric went even further by suggesting a possible military response.

Beijing has not been explicit about its potential responses. His military could try to stop Pelosi from landing in Taiwan or take other steps to prevent his visit, such as using fighter jets to intercept his US military plane.

Several people said the White House was trying to assess whether China was making serious threats or walking a tightrope in an attempt to pressure Pelosi into abandoning her trip.

US National Security Adviser Jake Sullivan and other senior National Security Council officials oppose the trip because of the risk of escalating tensions across the Taiwan Strait, according to two people familiar with the debate.

The NSC declined to say whether the administration had urged Pelosi to cancel her trip. John Kirby, the NSC’s strategic communications manager, said Friday that the NSC team provided “relevant geopolitical context, facts and information,” and that the president made her own decisions.

The controversy over the trip has raised concerns among Washington allies who fear it could trigger a crisis between the United States and China, according to several people familiar with the situation.

In another illustration of heightened concern, US Ambassador to China Nick Burns abruptly cut short a visit to Washington this week and returned to Beijing, in part because of growing concerns over Taiwan and also to prepare for an upcoming phone call between President Joe Biden and his Chinese counterpart Xi Jinping. The State Department declined to comment.

Biden said this week he expected to speak to Xi by the end of the month. The two leaders are expected to discuss Taiwan, which has become a serious flashpoint.

China has flown an increasing number of fighter jets into Taiwan’s “air defense identification zone” since Biden took office. In May, Biden said the United States would intervene militarily to defend Taiwan from any Chinese attack.

Controversy over Pelosi’s trip erupted after the Financial Times revealed she planned to travel to Taiwan to show her support as she comes under increasing pressure from China amid Russia’s invasion of China. Ukraine, heightening fears of Chinese military action. Pelosi and his delegation will also travel to Japan, Singapore, Indonesia and Malaysia.

The timing of the visit is sensitive for China. It will take place the same month as August 1, the anniversary of the founding of the People’s Liberation Army. It may also coincide with the annual Communist Party leadership conclave in the resort town of Beidaihe where cadres discuss politics but also sometimes grapple with power struggles.

The conclave is even more important this year as Xi will have to lay the groundwork for an unprecedented third term as party leader at the 20th Congress of the Communist Party of China in November.

Since the United States normalized relations with China and transferred diplomatic recognition from Taipei to Beijing in 1979, it has maintained a “one China” policy under which it recognizes Beijing as the sole government of the China while only acknowledging Beijing’s position that Taiwan is part of China.

Beijing has accused Biden of watering down that policy by taking steps such as sending a high-level delegation of former US officials to Taipei earlier this year.

Pelosi is believed to be the highest-ranking US politician to visit Taiwan since then-Republican President Newt Gingrich visited Taipei in 1997. Beijing opposes any move that appears to lend legitimacy to Taiwan in as an independent country or to make the American relationship more formal.

Some experts say China mistakenly believes the White House is coordinating the visit because Pelosi and Biden belong to the same party, even though Congress is independent and Biden lacks the power to block his travel plans.

The Pentagon briefed Pelosi this week on scenarios that could occur if she traveled to Taipei. Following that briefing, Biden told reporters that “the military thinks it’s not a good idea right now” for Pelosi to continue. But US officials said the military was content to describe the various risks associated with such a visit.

At a press conference the next day, Pelosi said Biden had raised no concerns about the trip, which she declined to confirm. However, she indirectly referenced it by saying that Biden seemed to point to some of the scenarios that could occur if she visited Taiwan.

“I think what the president was saying is [that] maybe the military was afraid our plane would be shot down or something by the Chinese,” she said. “I heard it anecdotally, but I didn’t hear it from the president.”

Pelosi’s office did not respond to a request for comment on whether she might abandon her trip.

Those briefed on national security affairs in Taipei said the risk of Beijing significantly escalating military aggression in response to Pelosi’s visit was more pronounced than last year given rising tensions.

“Previously, the gangster wore a suit, but now he pulls the knife straight out,” a senior Taiwan official said.