WASHINGTON — The United States warned China on Monday not to respond to an expected trip to Taiwan by Speaker Nancy Pelosi with military provocations even as American officials sought to reassure Beijing that such a visit would not be the first of its kind nor represent any change in policy toward the region.

With tensions rising on the eve of Ms. Pelosi’s anticipated arrival in Taipei, the White House said it was concerned that China might fire missiles into the Taiwan Strait, send warplanes into Taiwan’s air defense zone or stage large-scale naval or air activities that cross traditional lines.

“There is no reason for Beijing to turn a potential visit consistent with longstanding U.S. policy into some sort of crisis or conflict, or use it as a pretext to increase aggressive military activity in or around the Taiwan Strait,” John F. Kirby, a National Security Council spokesman, told reporters. “Meanwhile,” he added, “our actions are not threatening and they break no new ground. Nothing about this potential visit — potential visit, which oh, by the way, has precedent — would change the status quo.”

But Beijing made clear it was not reassured. “We would like to tell the United States once again that China is standing by, the Chinese People’s Liberation Army will never sit idly by, and China will take resolute responses and strong countermeasures to defend its sovereignty and territorial integrity,” Zhao Lijian, a Foreign Ministry spokesman, told reporters. “As for what measures, if she dares to go, then let’s wait and see.”

The standoff over the speaker’s visit has set nerves on edge on both sides of the Pacific at a time when the United States is already consumed with helping Ukraine fight off Russia’s invasion. Even as they were trying to head off a confrontation in Asia on Monday, Secretary of State Antony J. Blinken and other officials were announcing a new $550 million shipment of arms to Ukraine.

While military, intelligence and diplomatic officials who briefed Ms. Pelosi before she left for Asia cautioned that a stop in Taiwan might instigate a response that could escalate out of control, President Biden stopped short of urging her not to go out of deference to her status as head of a separate, equal branch of government.

In a telephone call with President Xi Jinping of China last week, Mr. Biden explained that he did not control Ms. Pelosi and, as a longtime former member of Congress himself, respected her right to make her own decisions. But American officials fear that China does not accept that he has no power to stop her.

Mr. Blinken stressed that point on Monday. “The speaker will make her own decisions about whether or not to visit Taiwan,” he said. “Congress is an independent, coequal branch of government. The decision is entirely the speaker’s.

He added that members of Congress routinely go to Taiwan, including earlier this year. “And so if the speaker does decide to visit, and China tries to create some kind of crisis or otherwise escalate tensions, that would be entirely on Beijing,” Mr. Blinken said. “We are looking for them, in the event she decides to visit, to act responsibly and not to engage in any escalation going forward.”

Ms. Pelosi, who arrived in Singapore on Monday, has not officially confirmed her plan to stop in Taiwan, citing security concerns. But local reports in Taiwan said officials there had been informed that she would arrive on Tuesday evening or Wednesday morning local time. She originally planned to visit Taiwan in April but called off that trip after testing positive for the coronavirus.

American officials monitoring intelligence reports have become convinced in recent days that China is preparing a hostile response of some sort — not an outright attack on Taiwan or an effort to intercept Ms. Pelosi’s plane, as some fear, but an assertion of military power that may go beyond even the aggressive encounters of recent months. Some cited the Taiwan Strait crisis of 1995 and 1996, when China fired missiles to intimidate the self-governing island and President Bill Clinton ordered aircraft carriers into area.

Analysts said a similar conflict could be vastly more perilous today because the People’s Liberation Army is far more robust than it was then, armed now with missiles that could take out carriers. The worry is that even if no combat is intended, an accidental encounter could easily spiral out of control.

“This is an exceptionally dangerous situation, perhaps more so than Ukraine,” said Evan Medeiros, a China expert at Georgetown University and a former Asia adviser to President Barack Obama. “The risks of escalation are immediate and substantial.”

At the White House, Mr. Kirby did not say whether American intelligence agencies had detected any concrete indications of Chinese actions, but he was unusually specific in outlining the possible responses that the United States anticipated.

White House officials have privately expressed concern that a visit by Ms. Pelosi would touch off a dangerous cycle of escalation in Asia at the same time Washington is already consumed with helping Ukraine fight off Russia’s invasion. Much of America’s military industrial complex is busy arming Ukraine, which could hamper efforts to bolster weapons shipments to Taiwan.

Mr. Kirby said American officials did not necessarily anticipate an attack by China in response but cautioned that the possible military demonstrations of force could touch off a conflict by mistake. “It does increase the risk of miscalculation, which could lead to unintended consequences,” Mr. Kirby said.

He seemed particularly intent on getting the message through to Beijing that it should not view any visit by Ms. Pelosi as a fresh provocation by the United States since she would not be the first speaker to go there; Speaker Newt Gingrich stopped in Taiwan in 1997. Mr. Kirby also stressed repeatedly that the United States still subscribed to its one-China policy of not recognizing independence for Taiwan.

“We’ve laid out very clearly if she goes — if she goes — it’s not without precedent,” he said. “It’s not new. It doesn’t change anything.”

While White House officials held out little hope of deterring Beijing, they opted to outline the possible Chinese responses to set the geopolitical ground in case there is a provocation so it will not come as a surprise.

But even if they get past the immediate conflict without escalation, officials worry that the dispute will accelerate an increasingly assertive posture by China, which has been moving in that direction in recent months anyway. Analysts said Mr. Xi cannot afford to look weak heading into a critical party congress in the fall when he will seek a third term.

Just as Mr. Xi’s domestic politics were a factor, so were Mr. Biden’s and Ms. Pelosi’s. Even if the speaker wanted to cancel her stop in Taiwan, it would be problematic at home because it would be viewed as an act of appeasement with a revanchist power. Republicans have been particularly vocal in encouraging her to proceed with the trip regardless of the Biden administration’s qualms.

Senator Marco Rubio, Republican of Florida, said that China should not push the United States over her trip. “I pray leaders of the Communist Party of #China will remember ancient but wise advice,” he wrote on Twitter, citing an aphorism, “When anger arises think of the consequences.”

“We may have deep domestic political differences,” he added, “but we will respond with unbreakable unity if threatened from abroad.”