China appears to have deployed surface-to-air missiles on a disputed island in the South China Sea, say reports.
Civilian satellite images taken on 14 February and published reported by Fox News appear to show two batteries of eight missile launchers and a radar system on Woody Island in the Paracels.
Taiwan, which along with Vietnam claims the island, confirmed the deployment.
The deployment of missiles would significantly increase tensions in the acrimonious South China Sea dispute.
China has been carrying out extensive land reclamation work in the South China Sea, 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 militarisation of the region.
Reuters quoted a US official confirming the "apparent deployment".
The latest images were captured by ImageSat International.
They show a close-up of a section of beach which resembles the coastline on the north of Woody Island, and point out two missiles batteries. Each battery is made up of four launchers and two control vehicles.
Two of the the launchers appear to have been erected, says the report.
An image taken of the site on 3 February shows the beach empty.
Fox News quoted a US defence official as saying the missiles appeared to be the HQ-9 air defence system, with a range of about 200km (125 miles).
Taiwan's defence ministry told the BBC the missiles would be capable of targeting civilian and military aircraft, and that it would watch developments closely.
The news comes as South East Asian regional leaders end a two-day meeting in Washington DC where South China Sea had been a topic of debate.
US President Barack Obama said the members had discussed the need for "tangible steps" to reduce tensions, including "a halt to further reclamation, new construction and militarization of disputed areas".
A Pentagon spokesman said the US could not comment on intelligence matters but watched such issues "very closely".
"The United States continues to call on all claimants to halt land reclamation, construction, and militarization of features in the South China Sea," he said in a statement to the BBC.
Rival countries have wrangled over territory in the South China Sea for centuries, but tension has steadily increased in recent years.
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.
Although largely uninhabited, the Paracels and the Spratlys may have reserves of natural resources around them. There has been little detailed exploration of the area, so estimates are largely extrapolated from the mineral wealth of neighbouring areas.
The sea is also a major shipping route and home to fishing grounds that supply the livelihoods of people across the region.