WASHINGTON (Reuters) - The United States is closely watching Chinese intentions toward Taiwan, concerned that Beijing’s growing military prowess may increase the risk it could one day consider bringing the self-ruled island under its control by force, a U.S. official said on Tuesday.

FILE PHOTO: Chinese military vehicles carrying DF-21D anti-ship ballistic missiles, potentially capable of sinking a U.S. Nimitz-class aircraft carrier in a single strike, travel past Tiananmen Gate during a military parade to commemorate the 70th anniversary of the end of World War II in Beijing Thursday Sept. 3, 2015. REUTERS/Andy Wong/Pool/File Photo

The senior U.S. defense intelligence official, who spoke to reporters on condition of anonymity, did not predict that China’s military, known as the People’s Liberation Army (PLA), would take such a step but said such a possibility was the top worry as China expands and modernizes its military capabilities.

“The biggest concern is that ... they are getting to a point where the PLA leadership may actually tell Xi Jinping that they are confident in their capabilities,” the official said, referring to China’s president.

Pressed on whether the official was referring to Chinese confidence in its capabilities to be able to successfully win a battle with Taiwan, the official said, “Well, specifically that would be the most concerning to me.”

Taiwan is only one of a growing number of flashpoints in the U.S.-China relationship, including a trade war between the countries, U.S. sanctions on the Chinese military, and China’s increasingly muscular military posture in the South China Sea.

However, in meetings with Pentagon leaders, PLA officials have long described Taiwan as China’s most sensitive issue.

China has repeatedly sent military aircraft and ships to circle the island on drills in the past few years and worked to isolate the island internationally, whittling down its few remaining diplomatic allies.

It has also strongly objected to U.S. warship passages through the Taiwan Strait this year, and issued a terse warning about Taiwan after talks in Beijing on Tuesday with the U.S. Navy’s top officer, Admiral John Richardson.

‘WHATEVER IT TAKES’

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In the talks, Chinese General Li Zuocheng, chief of China’s Central Military Commission Joint Staff Department, stressed that Taiwan was “China’s internal affairs” and that Beijing would allow “no external interference.”

"If someone tries to split Taiwan from China, the Chinese military will do whatever it takes to safeguard national reunification, national sovereignty and territorial integrity," according to an English-language statement here by China's defense ministry on the talks.

Washington has no formal ties with Taiwan but is bound by law to help it defend itself and is the island’s main source of arms. The Pentagon says Washington has sold Taiwan more than $15 billion in weaponry since 2010.

Xi has stepped up pressure on the democratic island since Tsai Ing-wen from the pro-independence Democratic Progressive Party became president in 2016.

On Jan. 2, Xi said in a speech that China reserved the right to use force to bring Taiwan under its control but would strive to achieve peaceful “reunification.”

Still, the U.S. defense intelligence official cautioned against over-reacting, noting Xi could believe he has plenty of time to achieve reunification with Taiwan.

The official also cautioned that China’s military still faced gaps in its capabilities.

“They could order them to go today, but I don’t think they’re particularly confident in that capability,” the official said.

Also on Tuesday, the Defense Intelligence Agency released a report describing Taiwan as the “primary driver” for China’s military modernization, which it said had made major advances in recent years.

U.S. defense officials have become particularly alarmed about China’s advances in super-fast “hypersonic” technology, which could allow it to field missiles that are far harder to detect.

"The result ... is a PLA on the verge of fielding some of the most modern weapon systems in the world. In some areas, it already leads the world," the report said here.