A top US naval commander has warned that China is "changing the operational landscape" in the South China Sea to control one of the world's busiest waterways.
Admiral Harry Harris, head of the US Pacific Command, told reporters that China is "clearly militarising" the disputed waters.
"You'd have to believe in a flat Earth to think otherwise," he said.
Admiral Harris made the comments as new satellite images appear to show that China has been building a series of radar facilities on islands closer to the shores of Vietnam, the Philippines and the island of Borneo than to China.
The images emerged only days after the US and Taiwan said China appeared to have deployed surface-to-air missiles on another island, prompting a blunt response from Beijing's Ministry of Defence, which said in a blog that China had established "necessary defence facilities" in the region that were "legal and appropriate".
"China's deployment of limited, necessary defence facilities on its own territory is its exercise of its right of self defence to which a sovereign state is entitled under international law," said Hua Chunying, a spokeswoman for the Chinese Foreign Ministry.
China has long claimed that its development of artificial islands was for civilian purposes, including weather observation, prompting Australia's foreign minister Julie Bishop to ask Beijing if other countries could have access to the facilities.
Chinese President Xi Jinping said last year that his country "does not intend to pursue militarisation" of the disputed Spratly island chain.
But Ian Storey, a senior fellow at the Institute of Southeast Asian Studies in Singapore, said China is seen to be constructing facilities for military, rather than civilian purposes.
"But that's how China will spin it," he told the New York Times.
Admiral Harris said China's deployments were escalating tensions in the region.
"I believe China seeks hegemony in East Asia," he said.
"Regrettably there are missiles and fighter aircraft and guns and other things that have been placed into the South China Sea and this of great concern to everyone who transits and relies on the South China Sea for peaceful trade."
But China's Foreign Minister Wang Yi said during a visit to Washington there was no problem with freedom of navigation of the shipping lanes.
He said militarisation was not the responsibility of one party alone, an apparent reference to US patrols.
"We don't hope to see any more close-up military reconnaissance, or the despatch of missile destroyers or strategic bombers in the South China Sea," he said.
According to the Center for Strategic and International Studies (CSIS) in Washington, the latest satellite images appear to show that China has built a high-frequency radar centre on Cuarteron Reef, more than 1000 kilometres south of China's southernmost province.
The US claims that China has constructed more than 1200 hectares of artificial land on reefs and shoals in the area in the last 19 months.
The CSIS said in a report last month the Chinese military build-up in the region meant the South China Sea "will be virtually a Chinese lake" by 2030.
Other claimants to parts of the waterways are Vietnam, the Philippines, Brunei, Malaysia and Taiwan.