Four US warships, including an aircraft carrier, have moved to waters east of Taiwan on “routine” deployments, as House of Representatives Speaker Nancy Pelosi heads to the island amid intensifying warnings from China.

The carrier USS Ronald Reagan had transited the South China Sea and is currently in the Philippines Sea, east of Taiwan and the Philippines and south of Japan, a US Navy official confirmed to Reuters on Tuesday.

The Japanese-based Reagan is operating with a guided missile cruiser, USS Antietam, and a destroyer, USS Higgins.

“While they are able to respond to any eventuality, these are normal, routine deployments,” the official, who spoke on the condition of anonymity, said. 

The official said the amphibious assault ship USS Tripoli was also in the area as part of a deployment to the region that started in early May from its home port of San Diego.

Pelosi, a long-time China critic, was expected to arrive in Taipei later on Tuesday, people briefed on the matter said, as the United States said it would not be intimidated by Chinese “sabre rattling” over the visit.

One person familiar with Pelosi’s itinerary said most of her planned meetings, including with President Tsai Ing-wen, were scheduled for Wednesday

Taiwan newspaper Liberty Times reported Pelosi’s delegation was due to arrive at 10.20pm on Tuesday (12.20am Wednesday AEST), without naming sources.

Pelosi visited 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.

Confirmation of the deployments comes as signs emerge of military activity on both sides of the Taiwan Strait ahead of Pelosi’s potential visit. 

In addition to Chinese planes flying close to the median line dividing the sensitive waterway on Tuesday morning, several Chinese warships had remained close to the unofficial dividing line since Monday, a source briefed on the matter told Reuters.

China’s defence and foreign ministries did not immediately respond to requests for comment. 

The source said Chinese warships and aircraft “squeezed” the median line on Tuesday morning, an unusual move the person described as “very provocative”. 

The person said Chinese aircraft repeatedly conducted tactical moves of briefly “touching” the median line and circling back to the other side of the strait, while Taiwanese aircraft were on standby nearby. 

Neither side’s aircraft normally cross the median line. 

Taiwan’s Defence Ministry said on Tuesday they have a full grasp of military activities near the island and will appropriately dispatch forces in reaction to “enemy threats” as tensions rise with China. 

The ministry had “reinforced” its combat alertness level until Thursday afternoon, the island’s official Central News Agency reported on Tuesday, citing unidentified sources.

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 and posted pictures online. The photographs have yet to be verified.

Chinese social media was abuzz with trepidation about potential conflict and patriotic fervour over the prospect of unification with Taiwan.

Since last week, China’s People’s Liberation Army has conducted various exercises, including live fire drills, in the South China, Yellow and Bohai seas.