BEIJING (AFP) – Beijing expressed anger Wednesday after reports the United States was considering ramping up its military presence in disputed South China Sea waters and confronting Chinese territorial claims with ships and aircraft.

Defense Secretary Ash Carter asked staff to explore sending Navy surveillance aircraft and vessels to islands which Washington believes have been rapidly built up by China in recent months, the Wall Street Journal said Tuesday, citing officials.

"We are severely concerned by relevant remarks made by the American side," Chinese foreign ministry spokeswoman Hua Chunying told a regular briefing in Beijing.

"Freedom of navigation does not mean that the military vessels or aircraft of a foreign country can willfully enter the territorial waters or airspace of another country."

China claims nearly all of the South China Sea, even waters approaching the coasts of its Asian neighbors, and has provoked alarm with increasingly bold actions.

United States officials last week accused China of building up to 800 hectares (2,000 acres) of artificial islands in the Spratlys, an archipelago of more than a hundred islands, reefs and atolls between Vietnam and the Philippines.

China could construct airfields, surveillance systems and harbors that would jeopardize regional stability, they said.

The US has so far not sent ships and aircraft within 12 nautical miles of the reclaimed reefs – the standard zone for territorial waters around natural land – in order to avoid escalating tensions, the WSJ said.

"We're just not going within the 12 miles - yet," it quoted a senior US official as saying.

A challenge by the US military in the region could potentially trigger a regional standoff, the newspaper added.

China urged "the relevant country" to "refrain from taking risky and provocative actions", Hua told reporters.

US Secretary of State John Kerry will meet senior officials in China later this week.

The US has used its military to push back against what it considers Beijing's aggressive stance before.

Last November two giant long-range B-52s flew over China's newly-declared Air Defense Identification Zone (ADIZ) in the East China Sea.