Chinese warplanes have soared close to Taiwan's maritime border ahead of US House of Representatives Speaker Nancy Pelosi's expected landing in Taipei later today.
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.
But this morning clips recorded in Xiamen, a coastal city facing Taiwan, show trains carrying military equipment, naval vessels and armoured trucks lining the roads.
A Chinese Foreign Ministry spokesperson says "collusion between the US and Taiwan occurred first" and that the US has ignored repeated warnings.
He added that countermeasures taken by Beijing will be "legitimate and necessary".
It comes as the US Navy deployed four warships to the Taiwan Strait.
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.
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.
In a statement on Tuesday, Taiwan's Defence Ministry said it has a full grasp of military activities near Taiwan and will appropriately dispatch forces in reaction to "enemy threats".
In the southeastern Chinese city of Xiamen, which lies opposite Taiwan and is home to a large military presence, residents reported sightings of armoured vehicles on the move on Tuesday and posted pictures online.
Chinese social media was abuzz with both trepidation about potential conflict and patriotic fervour over the prospect of unification with Taiwan, and the topic of Pelosi's visit was the top-trending item on the Twitter-like Weibo.
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 only 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 10:20 p.m. (1420 GMT) on Tuesday, without naming sources.
Pelosi was visiting Malaysia on Tuesday, having begun her Asia tour in Singapore on Monday. Her office said she will also go to South Korea and Japan, but 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.
Four sources said Pelosi was scheduled to meet a small group of activists who are outspoken about China's human rights record on Wednesday afternoon.
The meeting is likely to take place at the National Human Rights Museum at New Taipei City, according to a source with direct knowledge of the matter.
On Monday, Chinese foreign ministry spokesperson Zhao Lijian said 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.
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.
