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.”

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