TAIPEI, Taiwan (AP) — Taiwan President Tsai Ing-wen welcomed the former head of the U.S. Indo-Pacific Command, who had warned the island could face an invasion from China this decade, as the mainland ramps up pressure on the self-governing island.

Retired Adm. Phil Davidson, along with a group including colleagues from the U.S. think tank the National Bureau of Asian Research, arrived Monday in the capital Taipei following a string of delegate visits to Taiwan, which China claims as part of its territory, since tensions with the mainland spiked in August with the visit of then-U.S. Speaker Nancy Pelosi.

“I’m here to listen and learn from our hosts; thus far, I’ve done both,” he said. “I'm looking forward to continuing our discussions with President Tsai today and hearing her perspectives on the security environment and her outlook on U.S.-Taiwan relations.”

China’s People’s Liberation Army has stepped up its pressure on Taiwan in the past few years, sending navy vessels and fighter planes near the island.

Taiwan’s Defense Ministry said Tuesday that 20 Chinese aircraft crossed the central line in the Taiwan Strait — a long-time unofficial buffer zone between the sides, which separated during a civil war in 1949. It sent 14 other planes in nearby airspace.

A day later, Taiwan’s Defense Ministry said 17 Chinese aircraft crossed the median line Wednesday, with a total of 23 planes sent to fly around the island.

“Faced with the expansion of authoritarianism, Taiwan must strengthen our ability to defend ourselves,” she said, pointing to the upcoming extension of mandatory military service that she had just announced in December.

Tsai thanked Davidson on Thursday for his contributions to the “safety of the Taiwan Straits.”

Davidson, while still serving as the head of U.S. Indo-Pacific Command, had warned that China's claims over Taiwan were a rising and tangible threat.

“Taiwan is clearly one of their ambitions before then, and I think the threat is manifest during this decade — in fact, in the next six years,” he told the Senate Armed Forces Committee in 2021.

He clarified this remark while in Japan just before his visit to Taiwan, saying that the scenario would not necessarily be an all-out war.

“In my mind, that can be many lesser things than an all-out invasion. One of those would be the threats to outer islands, and I think it’s a grave security concern of Taiwan’s,” he told the Japan Times.

AP senior video producer Johnson Lai contributed to this report.