The report said that China had started building at least three intercontinental ballistic missile silo fields.
China has been expanding its nuclear arsenal much more quickly than predicted, narrowing the gap with the United States, the Pentagon said.
The number of Chinese nuclear warheads could increase to 700 within six years, and may top 1,000 by 2030, a report said on Wednesday.
It did not say how many weapons China has today, but a year ago the Pentagon said the number was in the “low 200s” and was likely to double by the end of this decade.
The United States, by comparison, has 3,750 nuclear weapons and has no plans to increase.
“The People's Liberation Army’s evolving capabilities and concepts continue to strengthen (China’s) ability to ‘fight and win wars’ against a ‘strong enemy,’ a likely euphemism for the United States,” the report said.
The assessment came in the US Department of Defense's annual report to Congress on Chinese military developments.
READ MORE: The United States’ nuclear defense is lagging behind Russia and China
Nuclear triad
China may have established what is known as a nuclear triad, the combination of land-, sea-, and air-based missiles that the United States and Russia have had for decades, the report said.
It said China is likely not seeking a capability to launch an unprovoked nuclear strike on a nuclear-armed adversary but wanted to deter attacks from others by maintaining a credible threat of nuclear retaliation.
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 defence official said, adding it "raises questions about their intentions."
READ MORE: The future of China-US relations
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.
The new US report said China's swift military modernisation aims to have the capability by 2027 to overcome any pushback to an effort to reclaim Taiwan, by pressure or military force.
In remarks shortly before the report's release on Wednesday, Milley told the Aspen Security Forum that the hypersonic missile test and other Chinese advances are evidence of what is at stake for the world.
“We are witnessing one of the largest shifts in global and geostrategic power that the world has witnessed,” he said.
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