A US warship on Tuesday sailed close to artificial islands Beijing is building in the South China Sea, a US defense official said, in a move likely to infuriate the Asian country.

A US warship on Tuesday sailed close to artificial islands Beijing is building in the South China Sea, a US defense official said, in a move likely to infuriate the Asian country.

The USS Lassen passed within 12 nautical miles of at least one of the land formations Beijing claims in the disputed Spratly Islands, the official told AFP.

Beijing slammed the US for sailing the warship near its artificial islands, saying the move was a "threat to China's sovereignty".

The USS Lassen "illegally entered" the waters near the islands, foreign ministry spokesman Lu Kang said in a statement, "without receiving permission from the Chinese government".

Under international law, a state's territorial waters extend 12 nautical miles from its shore. However, the US and others argue that this rule cannot apply to artificial islands.

"We are conducting routine operations in the South China Sea in accordance with international law," the official said. "We will fly, sail, and operate anywhere in the world that international law allows."

Tensions have mounted since China transformed reefs in the area into small islands capable of supporting military facilities, a move the US says threatens freedom of navigation.