The Pentagon says China has significantly increased its arsenal of short-range missiles facing Taiwan.
The estimate comes in a new Pentagon report to the US Congress on Chinese capabilities.
China is "exploring coercive strategies", the US report says
According to the Pentagon, China now has some 450 short-range missiles based opposite Taiwan.
That is an increase of 100 over last year's estimate.
In its latest report to the US Congress on China's military power, the Pentagon has also raised its estimate of China's production of such weapons by half - to 75 a year.
It says they are becoming more advanced and that China is developing a version which could reach Okinawa, Japan, where thousands of US troops are based.
Washington has in the past year resumed high-level military contacts with Beijing, but the Pentagon clearly remains concerned about China's military modernisation programme.
The latest report says preparing for a potential conflict across the Taiwan Strait is the primary driving force behind that modernisation and Pentagon officials have in the past described the missile build-up in particular as threatening.