China is deploying missiles in the disputed South China Sea to intimidate and coerce its neighbours, US Defence Secretary James Mattis says.
Speaking in Singapore, General Mattis said Beijing's actions called into question its broader goals.
He also said the issue of US troops in South Korea was "not on the table" at this month's summit between President Trump and North Korea's Kim Jong-un.
The US wanted complete denuclearisation of the Korean peninsula, he added.
South Korean Defence Minister Song Young-moo also told the Shangri-La Dialogue security summit that US forces in South Korea was a "separate issue from North Korea's nuclear issue".
There are currently about 28,500 US troops based in South Korea.
Mr Mattis told the security summit Beijing had deployed military hardware including anti-ship and surface-to-air missiles to locations across the South China Sea.
"Despite China's claims to the contrary, the placement of these weapon systems is tied directly to military use for the purposes of intimidation and coercion," General Mattis said.
General Mattis said the Trump administration wanted a constructive relationship with China but would compete vigorously if necessary.
The US recognised that China had a role to play in the region.
The South China Sea, a key trade route, is subject to overlapping claims by six countries.
China has been building small islands and other maritime features into military facilities there.
Last month China said it had for the first time landed bombers on Woody Island in the Paracel Islands, prompting US warnings that it was destabilising the region.
Woody Island, which China calls Yongxing, is also claimed by Vietnam and Taiwan.
The South China Sea dispute
- Sovereignty over two largely uninhabited island chains, the Paracels and the Spratlys, is disputed by China, Vietnam, the Philippines, Taiwan and Malaysia
- China claims the largest portion of territory, saying its rights go back centuries - in 1947 it issued a map detailing its claims
- The area is a major shipping route, and a rich fishing ground, and is thought to have abundant oil and gas reserves