China is on course to more than triple its stockpile of nuclear warheads to 1,500 by 2035, when it aims to complete the modernization of its military, the U.S. Defense Department said Tuesday in an annual report on the Asian country's military power.

The Pentagon also warned that Beijing aims to make the People's Liberation Army "a more credible military tool" by 2027 as it pursues unification with Taiwan, a self-ruled democratic island that Beijing views as its own.

Photo taken on Oct. 12, 2022, shows the Dong Feng-41 ICBM missile on display at an event held in Beijing to disseminate the achievements of China's leadership under President Xi Jinping in the past 10 years. (Kyodo) ==Kyodo

The latest report, mainly covering military developments involving China last year, reflects U.S. concerns over Beijing's continuing nuclear buildup and pressure on Taiwan, including increased flights in the island's self-declared air defense identification zone, which Washington views as "provocative and destabilizing."

According to the Defense Department, China "probably accelerated its nuclear expansion" in 2021 and its stockpile of nuclear warheads has surpassed 400, compared with an estimate a year earlier that the number of warheads possessed was in the low 200s.

While noting that the PLA plans to basically complete the modernization of its forces by 2035, the report, titled "Military and Security Developments Involving the People's Republic of China," said Beijing "will likely field a stockpile of about 1,500 warheads by its 2035 timeline."

A senior U.S. defense official said the latest estimate did not imply a major acceleration in the growth of the nuclear stockpile compared with the report last year, when the Pentagon assessed that China would likely possess at least 1,000 nuclear warheads by 2030.

But the official said the overall rapid buildup was "too substantial to keep under wraps."

"It does raise questions about whether they are...shifting away from a strategy that was premised on what they referred to as a 'lean and effective deterrent,' where they said they would have the kind of minimum number of nuclear weapons that was required for the PRC's national security," the official added, referring to the acronym for China's official name.

China has been reluctant to engage in arms control talks with Washington, asserting that the two countries possessing the largest nuclear arsenals -- the United States and Russia -- have the primary responsibilities toward nuclear disarmament.

According to the State Department, the U.S. stockpile of nuclear warheads consisted of 3,750 warheads as of September 2020.

On the security situation surrounding Taiwan, the report said China "intensified diplomatic, economic, political, and military pressure against Taiwan in 2021."

Tensions heightened this year as China was angered by a visit to Taiwan in August by U.S. House of Representatives Speaker Nancy Pelosi, the third-highest-ranking official in the United States. It was the first trip to the island in 25 years by a holder of the powerful congressional post.

With the PLA believed to be working toward three milestones for its military modernization -- 2027, 2035 and 2049 -- the report said the 2027 goal, if realized, "could give the PLA capabilities to be a more credible military tool" for the Chinese Communist Party to wield against Taiwan.

The PLA's centenary falls in 2027 and some U.S. military officers have said China is building its military capability toward being ready to invade and seize Taiwan around that year. The year 2049 is the 100th anniversary of the founding of People's Republic of China, with Beijing saying it will seek to achieve "national rejuvenation" by then.

The report also underscored that China "has never renounced the use of military force" over Taiwan, while noting that "the circumstances under which the PRC has historically indicated it would consider using force remain ambiguous and have evolved over time."

Military options that China could take against Taiwan may range from an air and maritime blockade to a full-scale amphibious invasion to seize and occupy some of its offshore islands or all of Taiwan, the Pentagon said.

It also said China's national strategy to seek "rejuvenation" is a "determined pursuit" to expand its national power and revise the international order in support of Beijing's system of governance and national interests, and the United States is viewed as presenting "obstacles" through efforts to contain the Asian country's rise.