China has increased the number of short-range ballistic missiles on its coast opposite Taiwan, the US has said.
The report says the military balance is tipping against Taiwan
In an annual report to Congress, the Pentagon claimed there were now up to 730 such missiles in place. Last year's report found only 500.
The Pentagon said China could now be spending up to $90bn a year on defence, and that its military build-up put regional balances at risk.
But China has dismissed the claims, insisting its rise would be peaceful.
"Not only is China not a threat to anyone, but we would also like to make friends with people in every country, work together and develop mutually beneficial co-operation in order to facilitate everyone's progress," Chinese Foreign Minister Li Zhaoxing said on Wednesday.
The Pentagon report said that China did not face a threat from any other nation.
Yet it found that Beijing continued to invest heavily in its military, and its modernisation plans are aimed primarily at winning a war with Taiwan.
REPORT'S KEY FINDINGS
Build-up of short-range missiles opposite Taiwan
Increase in long-range missiles and hardware from Russia
Military spending could be up to $90bn - the highest in Asia
But the US is the top military spender, with an estimated $399bn annual defence budget
According to the American findings, there are now between 650 and 730 short-range ballistic missiles in position opposite Taiwan, with 100 more being deployed every year.
China is also developing the capability to launch air strikes and mount a blockade against the island, the report said.
In the past, the US has cautioned both China and Taiwan not to change the status quo.
Washington is Taiwan's main arms supplier and could be drawn into any conflict.
But the Pentagon also believes that China's strategic planners are looking beyond Taiwan.
The report points to China's growing missile capability and the imminent deployment of mobile, long-range ballistic missiles, known as DF31s, which could hit targets worldwide with nuclear warheads.
The Pentagon report says Chinese defence spending could be up to $90bn this year, more than twice the estimated figure given by Beijing.
This would make it the largest military spender in Asia - and third in the world after the US and Russia.
The US itself is thought to have an approximate annual defence budget of almost $400bn, according to World Bank figures.
The Chinese navy has bought into service advanced guided missile destroyers, submarines and fighter aircraft, bought from Russia.
Over the long term, says the Pentagon, if current trends persist, the Chinese military could pose a credible threat to other modern militaries operating in the region.
According to the BBC's Pentagon correspondent Adam Brookes, this is code for American forces in Asia.
The drafting of this report has been a contentious process, reflecting divisions in Washington between those who view Chinese power as a serious emerging threat and those who take a more benign view, our correspondent says.
But the final product is a document tough in substance and in tone, which will do little or nothing to reassure those Americans who worry about China's intentions, he says.