China outlined a defense strategy on Tuesday to boost its naval capability farther from its shores, saying it faced a grave and complex array of security threats including in the disputed East Vietnam Sea.

In a policy document issued by the State Council, the country's cabinet, it vowed to continue growing its "open seas protection" and criticized neighbors who take "provocative actions" on what it claims are its reefs and islands.

The document comes as tensions rise over China's increasingly assertive posture in the disputed waters of the East Vietnam Sea, where Beijing has illegally engaged in land reclamation in Vietnam's Truong Sa (Spratly) archipelago.

China, which claims most of the East Vietnam Sea, criticized Washington after a U.S. spy plane flew over areas near the reefs last week, with both sides accusing each other of stoking instability in the region.

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

Yang Yujun, a spokesman for the Ministry of Defence, said in a briefing on Tuesday China's reclamation activities in Truong Sa were comparable with construction of homes and roads on its mainland.

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

He said some countries with "ulterior motives" had unfairly characterized China's military presence and sensationalized the issue. Surveillance activities in the region were increasingly common, and China would continue to take "necessary measures" to respond, Yang said.

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

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

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

It also said the military would strengthen security in areas critical to China's overseas interests, without giving details.