The Speaker of the US House of Representatives, Nancy Pelosi, arrived in Taipei, the capital of the island of Taiwan, which China considers part of its territory.
And Reuters news agency quoted a US source as saying that several Chinese warplanes flew near the dividing line at the Taiwan Strait.
China has warned more than once against Pelosi's visit to Taiwan, which it says is part of, while the United States said Monday that it would not be frightened by the Chinese "clatter of arms".
The source told Reuters that in addition to Chinese planes flying near the middle line of the strait, several Chinese warships have sailed near the unofficial dividing line since Monday. He added that Chinese warships and planes "crossed" the center line on Tuesday morning, an unusual move that the person described as "extremely provocative."
He said Chinese planes repeatedly made tactical moves by briefly "touching" the center line and returning to the other side of the strait on Tuesday morning, while Taiwanese planes were on standby nearby.
But he added that the Chinese planes left the area in the afternoon, but the ships remained.
Police in Taiwan confirmed the visit shortly before Pelosi's arrival on the 25-year-old self-ruled island.