"In recent months, China has undertaken destabilising, unilateral actions asserting its claims in the South China Sea," US Defense Secretary Chuck Hagel told an audience in Singapore recently, Reuters reported.

"China has restricted access to Scarborough Reef, put pressure on the long-standing Philippine presence at the Second Thomas Shoal, begun land reclamation activities at multiple locations, and moved an oil rig into disputed waters near the Paracel Islands," said Mr Hagel.

The US Defense Secretary's speech drew an angry response from China, which rebuked him for making "harsh, provocative" comments signalling a further deterioration in the already strained relations between the two countries.

According to Mr Hagel, the United States takes no position on territorial disputes in the South China Sea that pit China against Vietnam, the Philippines, Malaysia and Brunei.

But Washington said it will oppose any attempt to use "intimidation, coercion or the threat of force to assert those claims." It will also oppose any attempt to restrict over-flight and freedom of navigation.

Washington wants to uphold a "rules-based order" in which disputes are settled through diplomacy, well-established norms and international law.

China claims sovereignty over all the islands, rocks and reefs in the four main groups, while Vietnam, the Philippines, Malaysia and Brunei each claim some of them.

Much of the dispute revolves around the "Nine Dash Line," which appears on China's official maps and encompasses almost the entire South China Sea.

The Nine Dash Line made its first appearance in official atlases issued in 1948, though the territorial claims on which it rests go back much further. It has been subject to only minor modifications since then.