Beijing condemned the Philippines on Thursday over a Filipino warship grounded on a Chinese reef in the South China Sea.

It described the illegal grounding of the vessel on the Ren'ai Reef as a "serious encroachment of territorial sovereignty", and warned Manila not to stir up the situation in the South China Sea any further.

Observers said Beijing acted in response to an attempt by the Philippines to assert territorial claims by keeping the warship stranded on the reef since 1999.

"China's resolution and will to safeguard its territorial sovereignty is unswerving," Geng Yansheng, spokesman for the Ministry of National Defense, said at a press conference.

An allegation by the Philippines that Chinese vessels have threatened to cut off supplies of water and food to Philippine military staff at the reef is groundless, Geng said.

"Chinese naval patrols in the area are justifiable," he added.

After the warship was grounded on the reef, Beijing repeatedly asked Manila to retrieve it, but the Philippines ignored the request despite having promised to tow the ship away.

Li Guoqiang, deputy director of the Center for Chinese Borderland History and Geography at the Chinese Academy of Social Sciences, said, "The Philippine's logic is ludicrous in calling its grounded ship a symbol of occupation while it is in China's inherent territory."

China has indisputable sovereignty over the Nansha Islands, which include Ren'ai Reef and adjacent waters.

Manila has highlighted the situation on the reef at a time of heightened tension between the two countries. Last April, troops on a Philippine warship harassed Chinese fishermen in waters off China's Huangyan Island.

Foreign Ministry spokesman Hong Lei said on Thursday that Beijing has never tolerated Manila's illegal attempt to seize the reef and Chinese government vessels are entitled to patrol there.

He was speaking in response to a recent claim by the Philippines that Chinese ships pose a threat to the security of the Nansha Islands.

Hong said China also urged relevant countries to fully and sincerely implement the Declaration on the Conduct of Parties in the South China Sea, to refrain from actions that could aggravate or complicate the issue, and avoid any action that could undermine peace and stability in the region.

Yang Baoyun, a Southeast Asian studies expert at Peking University, said Manila is again acting like a victim but is actually provoking Beijing to gain international sympathy and show off its "influence" in the South China Sea.

"The Philippines also plans to play up the reef topic at the Shangri-La Dialogue, starting on Friday in Singapore, to win more support from major powers such as the United States. They did the same thing last year regarding the Huangyan Island," Yang said.

Li said that meetings of the Association of Southeast Asian Nations to be held later this year will be used by Manila as a platform for spreading the argument on the South China Sea issue.

China also rejected a report by the US Defense Science Board, which said Chinese hackers have gained access to the designs of two dozen US weapons systems.

The Ministry of National Defense said the accusation is a misjudgment that underestimates the Pentagon's security capability as well as Chinese people's wisdom.

Li Hong, secretary-general of the China Arms Control and Disarmament Association, said: "The accusation could be an excuse for the Pentagon to increase its military expenditure. And the cybersecurity issue, which has been exaggerated by the US media immediately before President Xi Jinping meets US President Barack Obama early next month, can become a bargaining chip for the US during the meeting."