In a policy document issued by the State Council, the Communist-ruled country's cabinet, China vowed to increase its "open seas protection", switching from air defence to both offence and defence, and criticised neighbours who take "provocative actions" on its reefs and islands.

China has been taking an increasingly assertive posture over recent years in the disputed waters of the South China Sea, where it has engaged in extensive land reclamation in the Spratly archipelago.

China claims most of the South China Sea and criticised Washington last week after a U.S. spy plane flew over areas near the reefs. Both sides accused each other of stoking instability.

A U.S. State Department spokesman declined to make a specific comment on the Chinese strategy paper, but said Washington urged Beijing "to use its military capabilities in a manner that is conducive to maintaining peace and stability in the Asia-Pacific region."

Jeff Rathke reiterated the U.S. view that China’s reclamation work had contributed to rising tensions and said building up of underwater features did not confer a right to a territorial sea or an exclusive economic zone.

White House spokesman Josh Earnest said President Barack Obama considered the South China Sea security situation "critically important" to U.S. national security and the global economy and said Washington was committed to working with other Asia-Pacific states to protect the free flow of commerce there.

While also declining to comment on the content of China's policy paper, Pentagon spokesman Colonel Steve Warren said its publication was "a step in the right direction" in terms of transparency and "exactly the type of thing that we’ve been calling for" in that respect.

China has overlapping claims with the Philippines, Vietnam, Malaysia, Taiwan and Brunei in the South China Sea, through which $5 trillion in ship-borne trade passes every year.

Chinese Defence Ministry spokesman Yang Yujun said China's reclamation in the Spratlys was comparable with construction of homes and roads on the mainland.

"From the perspective of sovereignty, there is absolutely no difference," he told reporters.

Some countries with "ulterior motives" had unfairly characterized China's military presence and sensationalised the issue, he said. Surveillance in the region was increasingly common and China would continue to take "necessary measures" to respond.

"Some external countries are also busy meddling in South China Sea affairs. A tiny few maintain constant close-in air and sea surveillance and reconnaissance against China," the strategy paper said in a thinly veiled reference to the United States.

OFFENCE AND DEFENCE

It said China's air force would shift its focus from territorial air defence to both offence and defence, and building airspace defences with stronger military capabilities.

China also announced plans for the building of two lighthouses in the South China Sea on Tuesday and broadcast a groundbreaking ceremony on state television, defying calls from the United States and the Philippines for a freeze on such activity.

Foreign Ministry spokeswoman Hua Chunying said the construction was to help maritime search and rescue, disaster relief, environmental protection and navigational security.

Wu Shicun, president of the government-affiliated National Institute for South China Sea Studies, said the lighthouses were among the first of planned civilian-use facilities in the region.

"The reefs are located near an important commercial shipping route, so there will be continued development to maintain the security of those shipping lanes," he told Reuters.

The strategy paper also said the People's Liberation Army's nuclear force, known as the Second Artillery Corps, would strengthen its capabilities for deterrence and nuclear counterattack as well as medium- and long-range precision strikes.

"China faces many complex maritime security threats and challenges and requires a navy that can carry out multifaceted missions and protect its sovereignty," Wang Jin, a senior colonel, told reporters.

The paper also cited "grave threats" to China's cyber infrastructure, adding that China would hasten development of a cyber military force.

Self-ruled Taiwan, which China considers a renegade province, called on all South China Sea claimants to shelve their disagreements to enable talks on sharing resources before a conflict breaks out.

Japan meanwhile will join a major U.S.-Australian military exercise for the first time in a sign of growing security links between the three countries as tensions fester over China's moves.

All three nations have said they are concerned about freedom of movement through the South China Sea and air space.

China's Ministry of Defence said on Tuesday it had carried out military training for party cadres from border and coastal areas on border defence, among other topics.

The trainees, who visited military combat units, developed a better understanding of the "national security situation", said a statement on the ministry's website.