WASHINGTON: Two US B-52 bombers flew close to disputed islands in the South China Sea claimed by Beijing and were given verbal warnings from a Chinese air traffic controller, the Pentagon said Thursday.

It is just the latest challenge from Washington to Beijing over the fate of the Spratly Islands, after the guided missile destroyer the USS Lassen last month sailed past a series of islets in Subi Reef in the Spratly chain.

It also comes ahead of a visit to Asia next week by President Barack Obama that will see territorial disputes at the fore of discussions with regional leaders, several of whom have claims in the resource-rich South China Sea.

"On a flight that took off and returned to Guam on November 8 and 9 respectively, two B-52s flying a routine mission in international airspace in the vicinity of the Spratly Islands in the South China Sea received two verbal warnings from a Chinese ground controller despite never venturing within 15 nautical miles of any feature," said Commander Bill Urban, a Pentagon spokesman.

"Both aircraft continued their mission without incident, and at all times operated fully in accordance with international law."

Washington has said that China's transformation of geographical features in the Spratlys into artificial islands capable of supporting military facilities poses a threat to freedom of navigation in the critical area.

Beijing has repeatedly insisted that it protects such freedoms, while claiming sovereignty over almost the whole of the South China Sea, even areas close to the coasts of other states.

Speaking to reporters Friday, Chinese foreign ministry spokesman Hong Lei said Beijing opposes "the action of undermining China's sovereignty and security under the pretext of freedom of navigation and overflight."

Also read: US flies B-52 bombers in China’s air defence zone