Taiwan's defence ministry has 'reinforced' its combat alertness level from Tuesday morning to Thursday noon, the island's official central news agency reported, citing unidentified sources.
US House of Representatives Speaker Nancy Pelosi was set to visit Taiwan on Tuesday, three sources said, as the United States said it wouldn't be intimidated by Chinese threats to never "sit idly by" if she made the trip to the self-ruled island claimed by Beijing.
Meanwhile, several Chinese warplanes flew close to the median line of the sensitive Taiwan Strait on Tuesday morning, a source briefed on the matter told Reuters.
The source said several Chinese warships have stayed close to unofficial dividing line since Monday, adding that Taiwan had dispatched aircraft to monitor the situation.
Taiwan's defence ministry was not immediately able to respond to a request for comment.
The source said both Chinese warships and aircraft "squeezed" the median line on Tuesday morning, an unusual move the person described as "very provocative." Neither side's aircraft normally cross the median line.
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.
On Monday, US Secretary of State Antony Blinken said that a potential visit to Taiwan by House of Representatives Speaker Nancy Pelosi would be entirely her decision, but called on China not to escalate tensions in the event of a visit.
"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," Blinken said after nuclear nonproliferation talks at the United Nations.
"We are looking for them (China) - in the event she decides to visit - to act responsibly and not to engage in any escalation going forward."
China claims democratically-ruled Taiwan as its own territory and has ramped up military and political pressure to try and force the island to accept Chinese rule.
Taiwan rejects China's claims and vows to defend itself.