TAIPEI (REUTERS) – Several Chinese warplanes flew close to the median line of the sensitive Taiwan Strait on Tuesday (Aug 2) morning, a source briefed on the matter told Reuters, as tensions mounted on news that US House of Representatives Speaker Nancy Pelosi was set to visit the island.

The source said several Chinese warships have stayed close to the 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.

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.

Separately, Taiwan’s official Central News Agency, citing unidentified sources, reported that the island’s defence ministry has “reinforced” its combat alertness level from Tuesday morning to Thursday noon.

The latest development comes as the US said earlier that it would not be intimidated by Chinese threats to never “sit idly by” if Mrs Pelosi made the trip to the self-ruled island claimed by Beijing.

This story is developing.