US Secretary of State John Kerry slammed China for its increased "militarization" in the strategically important South China Sea, after Beijing deployed surface-to-air missiles on a disputed island there.

US Secretary of State John Kerry slammed China for its increased "militarization" in the strategically important South China Sea, after Beijing deployed surface-to-air missiles on a disputed island there.

"There is every evidence, every day, that there has been an increase of militarization of one kind or another. It's of a serious concern," Kerry told reporters on Wednesday.

A US official said that China has deployed surface-to-air missiles on Woody Island, part of the Paracel Islands chain.

The official said the missiles appeared to be HQ-9s, which have a range of about 125 miles (200 kilometers).

Experts say they could be used to target enemy aircraft.

Fox News first reported missile launchers and a radar system had arrived on Woody Island in recent days, referring to satellite imagery. Taiwan's defense ministry later confirmed the facility's existence.

"We believe the photos are accurate and that China has deployed SAMs to Woody Island," the US official said.

Beijing has controlled all of the Paracels, which are also claimed by Hanoi and Taipei, since seizing several from South Vietnam in a brief, bloody battle towards the end of the Vietnam War.

But tensions in the sea have mounted in recent months since China transformed contested reefs in the Spratly islands further south into artificial islands capable of supporting military facilities.