China has branded as "irresponsible" US comments that it is intimidating its neighbours with its military deployment in the South China Sea.
A top Chinese general said China had the right to deploy troops and weapons "on its own territory".
Earlier US Defence Secretary James Mattis said Beijing's actions called into question its broader goals.
Six countries have competing claims in the sea, but China has backed its own with island-building and patrols.
Gen Mattis had made his critical comments at a security summit in Singapore.
Speaking at the same conference, China's Lt Gen He Lei said: "Any irresponsible comments from other countries cannot be accepted."
Gen He said Beijing's deployments were part of a policy of "national defence", adding: "They are for the purpose of avoiding being invaded by others.
"As long as it is on your own territory you can deploy the army and you can deploy weapons."
Gen He added: "We see any other country that tries to make noise about this as interfering in our internal affairs."
Gen Mattis said Beijing had deployed military hardware, including anti-ship missiles, surface-to-air missiles and electronic jammers 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," he said.
"China's policy in the South China Sea stands in stark contrast to the openness that our strategy promotes, it calls into question China's broader goals."
Despite his criticism, Gen Mattis added that the US would "continue to pursue a constructive, results-oriented relationship with China" with "co-operation whenever possible".
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.
Gen Mattis was speaking just 10 days before President Donald Trump is scheduled to meet North Korean leader Kim Jong-un in Singapore.
Gen Mattis said the issue of removing US troops from South Korea was "not on the table" and that "our objective remains the complete, verifiable, and irreversible denuclearisation of the Korean peninsula".
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