Four US warships, including an aircraft carrier, were positioned in waters east of Taiwan on what the US Navy called routine deployments on Tuesday amid Chinese anger over US House of Representatives Speaker Nancy Pelosi's visit the island.
The carrier USS Ronald Reagan had transited the South China Sea and was currently in the Philippines Sea, east of Taiwan and the Philippines and south of Japan, a US Navy official said, reports Reuters.
The Japan-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," said the official, who spoke on the condition of anonymity.
They would not comment on precise locations.
The official said the amphibious assault ship USS Tripoli was also in the area.
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 warnings over the visit.
Signs emerged of military activity on both sides of the Taiwan Strait ahead of Pelosi's visit.
Chinese planes flew close to the median line dividing the waterway on Tuesday morning and 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 the 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 - a move they described as provocative.
The person said three other Chinese warships on Tuesday carried out drills to simulate attacks on carrier-borne aircraft in waters east of Taiwan. The ships had been tracked sailing through Japan's southern islands at the weekend, the Japanese Defence Forces said.
Neither side's aircraft normally cross the median line.
Taiwan's Defense Ministry said it would appropriately send forces in reaction to "enemy threats".
The ministry had reinforced its combat alertness level from Tuesday morning to Thursday noon, the official Central News Agency reported, citing unidentified sources.
Since last week, the People's Liberation Army has conducted various exercises, including live fire drills, in the South China, Yellow Sea and Bohai Seas.
Some regional military analysts say that increased deployments at a time of tension raise the risk of accidents, even if no side wants an actual conflict.