U.S. Secretary of Health and Human Services Alex Azar arrived in Taiwan on Sunday (August 9), the highest-level U.S. official to visit the island in four decades.
China decried the trip. Beijing claims Taiwan as its own.
"China firmly opposes official interactions between the U.S. and Taiwan and this position has always been clear. China has made stern representations to the U.S. in Beijing and Washington."
Beijing has threatened unspecified countermeasures to Azar's visit.
Azar arrived at Taipei's downtown Songshan airport on a U.S. government aircraft, and was met by Brent Christensen, the de facto U.S. ambassador to Taiwan, and by Taiwan Deputy Foreign Minister Tien Chung-kwang.
In accordance with COVID-19 rules, there were no handshakes and all officials wore masks, including Azar.
Washington broke off official ties with Taipei in 1979 in favor of Beijing.
The Trump Administration has made strengthening its support for the democratic island a priority, and boosted arms sales.
Azar's trip comes as China-U.S. relations hit a low point over trade, the handling of the coronavirus, and human rights.
On Monday he will sign a health cooperation memorandum of understanding with Taiwan's government and visit Taiwan's Centres for Disease Control.
He is also scheduled to meet President Tsai Ing-wen during his visit.