US Secretary of State John Kerry says Washington is seriously concerned about increased Chinese militarisation in the contested South China Sea.
He was responding to reports Beijing has deployed surface-to-air missiles on a disputed island in the region.
China dismissed the reports as "hype", but said it had the right under international law to defend itself.
Several nations claim territory in the resource-rich South China Sea, which is also an important shipping route.
A spokesman for Mr Kerry said satellite images appeared to confirm China had deployed anti-aircraft missiles on Woody or Yongxing Island in the Paracels.
The island is claimed by China, Taiwan and Vietnam and the presence of missiles would significantly increase tensions.
Mr Kerry said the US expected to have a "very serious conversation" with China over its presence.
"There is every evidence, every day, that there has been an increase of militarisation from one kind or another. It's a serious concern," he said.
The latest images of Woody Island were captured by ImageSat International.
A picture dated 3 February shows a beach on the island empty. By 14 February it contains several missile launchers and support vehicles.
But the Chinese Foreign Minister, Wang Yi, said reports were a Western media invention.
He defended "the limited and necessary self-defence facilities" on islands inhabited by Chinese personnel as "consistent with the right for self-preservation and self-protection.... under the international law".
China has been carrying out extensive land reclamation work in the region, which it says is legal and for civilian purposes.
But the work has angered other countries which also claim the territory, and there is growing concern about the implications of the area becoming militarised.
The South China Sea dispute has been a topic of debate at a meeting of South East Asian regional leaders in California.
US President Barack Obama said the members had discussed the need for "tangible steps" to reduce tensions.
What is the South China Sea dispute?Rival countries have wrangled over territory in the South China Sea for centuries, but tension has steadily increased in recent years.
Its islets and waters are claimed in part or in whole by Taiwan, China, Vietnam, the Philippines, Malaysia and Brunei.
China has backed its expansive claims with island-building and naval patrols, while the US says it opposes restrictions on freedom of navigation and unlawful sovereignty claims - by all sides, but seen by many as aimed at China.
The frictions have sparked concern that the area is becoming a flashpoint with global consequences.
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