US House of Representatives Speaker Nancy Pelosi was expected to arrive in Taipei later on Tuesday, people briefed on the matter said, as several Chinese warplanes flew close to the median line dividing the Taiwan Strait, a source told Reuters.
China has repeatedly warned against Pelosi going to Taiwan, which it claims as its own, and the United States said on Monday that it would not be intimidated by Chinese "sabre rattling" over the visit.
In addition to Chinese planes flying close to the median line of the sensitive waterway on Tuesday morning, several Chinese warships had remained close to the unofficial dividing line since Monday, the source told Reuters.
The source said both Chinese warships and aircraft "squeezed" the median line on Tuesday morning, an unusual move the person described as "very provocative."
The person said the Chinese aircraft repeatedly conducted tactical moves of briefly "touching" the median line and circling back to the other side of the strait on Tuesday morning, while Taiwanese aircraft were on standby nearby.
Neither side's aircraft normally cross the median line.
Taiwan's defence ministry was not immediately able to respond to a request for comment.
Taiwan's defence ministry had "reinforced" its combat alertness level from Tuesday morning to Thursday noon, the island's official Central News Agency reported on Tuesday, citing unidentified sources.
CNA said the alertness level of Taiwan's armed forces is roughly categorised into "peace time" and "war time", and at the moment the level stayed at peace time, but it would be "appropriately adjusted" according to the level of threat.
One person familiar with Pelosi's itinerary said that most of her planned meetings, including with President Tsai Ing-wen, were scheduled for Wednesday, and that it was possible that her delegation would arrive in Taiwan early on Wednesday.
"Everything is uncertain," the person said.
Taiwan newspaper Liberty Times said Pelosi's delegation was due to arrive at 10:20pm (1420 GMT) on Tuesday, without naming sources.
Pelosi was visiting Malaysia on Tuesday, having begun her Asia tour in Singapore on Monday. Her office has said she will also go to South Korea and Japan, but has made no mention of a Taiwan visit.
Taiwan's foreign ministry said it had no comment on reports of Pelosi's travel plans, but the White House - which would not confirm the trip - said she had the right to go.
Beijing's responses could include firing missiles near Taiwan, large-scale air or naval activities, or further "spurious legal claims" such as China's assertion that the Taiwan Strait is not an international waterway, White House national security spokesperson John Kirby told reporters in Washington on Monday.
"We will not take the bait or engage in sabre rattling. At the same time, we will not be intimidated," Kirby said.
The Taiwan dollar slipped to its lowest levels in more than two years on the weaker side of 30 per US dollar and investor worries about a potential Pelosi visit and China's reaction appeared to be behind Tuesday's declines in China's yuan and north Asian stock markets.
One source told Reuters that the United States had informed some allies about Pelosi's visit to Taiwan. Two other sources said Pelosi was scheduled to meet a small group of activists who are outspoken about China's human rights record during her stay in Taiwan, possibly on Wednesday.
Chinese foreign ministry spokesperson Zhao Lijian told a regular daily briefing on Monday that it would be "a gross interference in China's internal affairs" if Pelosi visits Taiwan, and warned that "the Chinese People's Liberation Army will never sit idly by."
Asked what kind of measures the PLA might take, Zhao said: "if she dares to go, then let us wait and see."
China views visits by US officials to Taiwan, a self-ruled island claimed by Beijing, as sending an encouraging signal to the pro-independence camp in the island. Washington does not have official diplomatic ties with Taiwan but is bound by US law to provide the island with the means to defend itself.
A visit by Pelosi, who is second in the line of succession to the US presidency and a long-time critic of China, would come amid worsening ties between Washington and Beijing.
A video by the People's Liberation Army's Eastern Theater Command, which showed scenes of military exercises and preparations and was posted on state media sites on Monday evening, urged troops to "stand by in battle formation, be ready to fight upon command, bury all incoming enemies."
The White House has dismissed China's rhetoric as groundless and inappropriate.
Kirby said that nothing about Pelosi's possible trip changed US policy toward Taiwan, and that Beijing was well aware the division of powers within the US government meant Pelosi would make her own decisions about the visit.
"The speaker has the right to visit Taiwan," he told the White House briefing.
During a phone call last Thursday, Chinese President Xi Jinping warned US President Joe Biden that Washington should abide by the one-China principle and "those who play with fire will perish by it".
Biden told Xi that US policy on Taiwan had not changed and that Washington strongly opposes unilateral efforts to change the status quo or undermine peace and stability across the Taiwan Strait.
Beijing considers Taiwan to be part of its territory and has never renounced using force to bring the island under its control. Taiwan rejects China's sovereignty claims and says only its people can decide the island's future.