US will fly and sail by Spratlys whenever it likes, says Defence Secretary Carter in rebuff to Beijing over disputed South China Sea

The United States military would sail and fly wherever international law allowed, including the disputed South China Sea, US Defence Secretary Ash Carter said in a rebuff to China.

Carter spoke after a two-day meeting between US and Australian foreign and defence ministers at which the long-time allies agreed to expand defence cooperation and expressed "strong concerns" over Beijing's building on disputed islands.

"Make no mistake, the United States will fly, sail and operate wherever international law allows, as we do around the world, and the South China Sea will not be an exception," Carter said. "We will do that in the time and places of our choosing."

Carter had been asked about reports that the US had already decided to conduct freedom-of-navigation operations inside 12 nautical mile limits that China claims around islands built on reefs in the Spratly archipelago.

In Beijing, when asked about Carter's remarks, Foreign Ministry spokeswoman Hua Chunying said China had not militarised the South China Sea.

"I want to point out that some countries, in a region far from their own lands, have deployed offensive weaponry on a large scale and flexed their military muscles again and again in the South China Sea," Hua said.