TAIPEI: 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.

U.S. 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.

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, as tensions mounted on news Pelosi was set to visit Taiwan during the day.

The source said several Chinese warships have stayed close to unofficial buffer in the waterway 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.