TAIPEI, Taiwan (Reuters) — Chinese air force jets briefly crossed the mid-line of the Taiwan Strait on Monday and were tracked by Taiwanese missiles, Taiwan’s government said, as U.S. Secretary of Health and Human Services Alex Azar visited the island to offer President Donald Trump’s support.

Azar arrived in Taiwan on Sunday, the highest-level U.S. official to visit in four decades.

China, which claims the island as its own, condemned the visit which comes after a period of deteriorating relations between China and the United States.

China, which had promised unspecified retaliation to the trip, flew J-11 and J-10 fighter aircraft briefly onto Taiwan’s side of the sensitive and narrow strait that separates it from its giant neighbor, at around 9 a.m. (local time), shortly before Azar met Taiwan President Tsai Ing-wen, Taiwan’s air force said.

The aircraft were tracked by land-based Taiwanese anti-aircraft missiles and were “driven out” by patrolling Taiwanese aircraft, the air force said in a statement released by the defense ministry.

China’s defense ministry did not immediately comment.

A senior Taiwan official familiar with the government’s security planning told Reuters that China was obviously “targeting” Azar’s visit with a “very risky” move given the Chinese jets were in range of Taiwan’s missiles.

The incursion was only the third time since 2016 that Taiwan has said Chinese jets had crossed the strait’s median line.

The Trump administration has made strengthening its support for the democratic island a priority, amid deteriorating relations between Washington and Beijing, and has boosted arms sales.

“It’s a true honor to be here to convey a message of strong support and friendship from President Trump to Taiwan,” Azar told Tsai in the Presidential Office, standing in front of two Taiwanese flags.

Washington broke off official ties with Taipei in 1979 in favor of Beijing.

(Reporting by Ben Blanchard and Yimou Lee; Editing by Lincoln Feast, Robert Birsel)