China could have 700 deliverable nuclear warheads by 2027, and could top 1,000 by 2030 -- an arsenal two-and-a-half times the size of what the Pentagon predicted only a year ago, according to the report.

The People's Republic of China (PRC) "is investing in, and expanding, the number of its land-, sea-, and air-based nuclear delivery platforms and constructing the infrastructure necessary to support this major expansion of its nuclear forces.

The assessment came in the US Department of Defense's annual report to Congress on Chinese military developments.

Like the United States and Russia, the two leading nuclear powers, China is building a "nuclear triad," capabilities to deliver nuclear weapons from land-based ballistic missiles, from missiles launched from the air, and from submarines, it said.

The report said China is likely not seeking a capability to launch an unprovoked nuclear strike on a nuclear-armed adversary -- primarily the United States -- but wanted to deter attacks from others by maintaining a credible threat of nuclear retaliation.

A year ago, the Pentagon's China report said the country had about 200 deliverable warheads and would double that by 2030.

Independent researchers have in recent months published satellite photographs of new nuclear missile silos in western China.

The acceleration "is very concerning to us," a US defense official said.

It "raises questions about their intentions," the official said, calling for more transparency from Beijing over its nuclear force development.

- Main US rival -

The Pentagon has declared China its principal security concern for the future, as Beijing undertakes to build the People's Liberation Army into "world-class forces" by 2049, according to its official plan.

China is expanding its air, space and sea forces with the aim of projecting its power globally, much as the United States military has for decades.

The rivalry has increased concerns about a possible clash between the US and China, especially over Taiwan, which China claims as its territory but which is closely supported by the Untied States.

The new US report said China's swift military modernization aims to have the capability by 2027 to overcome any pushback to an effort to reclaim Taiwan, by pressure or military force.

By 2027, the report says, China aims to have "the capabilities to counter the US military in the Indo-Pacific region, and compel Taiwan's leadership to the negotiation table on Beijing's terms."

- October 2020 crisis -

The report confirmed news stories in recent months saying that in October 2020 Pentagon officials were forced to quell real concerns in Beijing that the United States, driven by domestic political tensions related to the presidential election, intended to instigate a conflict with China in the South China Sea.

Underscoring its fears, the PLA had issued intensified warnings in state-controlled media, launched large scale military exercises, expanded deployments and put troops on heightened readiness, the report said.

After senior Pentagon officials moved to directly speak to Chinese counterparts, the concerns eased and a Chinese defense spokesman announced publicly that the United States in fact did not plan to trigger a crisis.

"These events highlighted the potential for misunderstanding and miscalculation, and underscored the importance of effective and timely communication," the report said.

The report also questioned the PLA's intent in biological research into substances that potentially have both medical and military uses.

"Studies conducted at PRC military medical institutions discussed identifying, testing, and characterizing diverse families of potent toxins with dual-use applications," the report said, raising concerns over compliance with global biological and chemical weapons treaties.

Such concerns have echoed since early 2020 after the Covid-19 pandemic erupted first in the area of a Chinese biological research laboratory with PLA connections in Hunan.

The Chinese have denied the lab had anything to do with the Covid outbreak, but have limited access to it from investigators.