Taiwan's defence ministry said Thursday that a Chinese anti-submarine helicopter and three warships had been detected around the self-ruled island, after President Tsai Ing-wen met US House Speaker Kevin McCarthy in Los Angeles.
The statement came after Tsai met McCarthy in California during a stopover on her way back from a trip to Guatemala and Belize, which are among Taiwan's only 13 diplomatic allies.
China had threatened a "resolute" response to the meeting.
Taipei's Mainland Affairs Council, the island's top China policy-making body, on Wednesday accused Beijing of "obstructing" trade in the Taiwan Strait with on-site inspections on cargo and passenger ships.
Chinese maritime authorities had earlier said they were stepping up patrols in the waters separating the island from mainland China without providing more details.