The United States responds to Beijing's warning to the US over Taiwanese President Tsai Ing-wen's US visit by saying China should not use Tsai's stopover "as a pretext to step up any aggressive activity around the Taiwan Strait."

"There's no reason for them to react harshly," White House National Security spokesman John Kirby tells reporters.

"This transit is consistent with our longstanding unofficial relationship with Taiwan and it is consistent with the United States' One China policy, which remains unchanged," Kirby adds.