The armed forces have vowed to "fulfill their duty" to safeguard China's territory in the South China Sea, a Defense Ministry spokesman said on Thursday.

"China's military forces will collaborate closely with related governing bodies, including fishery administration and maritime law enforcement, to jointly ensure the country's maritime rights and interests," Geng Yansheng said in Beijing.

This was the first official remark from the armed forces following a standoff with a Philippine warship in waters off China's Huangyan Island on April 10.

Analysts said the comments were also in response to growing domestic demand to ensure sovereignty in the South China Sea.

Earlier, Defense Minister Liang Guanglie said that any military action will be based on the needs of diplomacy.

Media reports said that China has sent a nuclear-powered submarine to the South China Sea, but the spokesman did not confirm or deny the accuracy of the reports.

Beijing has shown reason and restraint in handling the dispute and tried to calm tension by withdrawing patrol ships, yet Manila's recent decision to send more vessels to the waters worsens the situation, Yang Baoyun, a professor at Peking University, said.

"In the meantime, China should beef up infrastructure, tourism and administration on islands in the South China Sea to avoid further disputes," Yang added.

Manila's standoff against Beijing has entered the seventeenth day and Beijing has not sent warships to the area.

Huangyan Island has been an integral part of China's territory for centuries and the Philippines admitted China's sovereignty over the island before it started to officially lay rival claims in 1997.

On April 10, 12 Chinese fishing boats were harassed by a Philippine warship while taking refuge from stormy weather in a lagoon near the island. Two Chinese patrol ships in the area later came to the fishermen's rescue, and the warship left.

The Chinese fishermen returned home, but the standoff continues. Philippine vessels were reportedly still in China's territorial waters on Thursday.

"China's military shoulders the mission of guarding the country's territorial sovereignty as well as maritime rights and interests, and it has persisted in fulfilling its duty under the country's unified deployment," Geng added.

Beijing has reiterated its willingness to settle the dispute through diplomatic consultation and the Foreign Ministry on Thursday, once again, called Manila to "jointly work to de-escalate" the situation.

Manila bases its claim over the island by saying that the island is "within" its so-called exclusive economic zone.

Beijing believes the Manila claim is both "groundless" and not applicable to the United Nations Convention on the Law of the Sea.

Liu also rejected Manila's suggestion to take the dispute to the International Tribunal for the Law of the Sea. Taking the dispute to international courts is "totally out of the question", said Liu, as unilateral action will not resolve the situation.

Meanwhile, the Philippines has reiterated its plan to discuss the dispute with Washington during US-Filipino talks, scheduled to start on April 30 in Washington.

Manila said on Thursday that it would seek more US military help during the top-level talks.

The Philippine claim and its tough stance on the South China Sea stems partly from its alliance with the US, said Chu Hao, a researcher in Southeast Asia studies at the China Institute of Contemporary International Relations.

"A tighter bond with the US instead of China is now a mainstream of Manila's policy making," Chu said.

The 12-day US-Philippine joint military exercise, Balikatan, is scheduled to wrap up on Friday.

The US is beefing up its military presence in the South China Sea and this will make the regional situation more complex, China's Liberation Army Daily warned last week.

Defense Minister Liang also said that Washington should do more to contribute to the peace and stability of the Asia-Pacific region.

Cui Haipei in Beijing contributed to this story.