China has begun combat-ready patrols in the waters around a disputed group of islands in the South China Sea, the Defence Ministry said on Thursday, the latest escalation in tension over the potentially resource-rich area.

Asked about what China would do in response to Vietnamese air patrols over the Spratly Islands, the ministry's spokesman, Geng Yansheng, said China would "resolutely oppose any militarily provocative behavior."

"In order to protect national sovereignty and our security and development interests, the Chinese military has already set up a normal, combat-ready patrol system in seas under our control," he said.

"The Chinese military's resolve and will to defend territorial sovereignty and protect our maritime rights and interests is firm and unshakeable," Mr. Geng added, according to a transcript on the ministry's website of comments at a briefing.

He did not elaborate. The ministry does not allow foreign reporters to attend its monthly briefings.

China is involved in long-running disputes with Vietnam and the Philippines about ownership of the South China Sea and its myriad, mostly uninhabited, islands and atolls. Taiwan, Malaysia and Brunei also have claims.

Last week, China said it "vehemently opposed" a Vietnamese law asserting sovereignty over the Paracel and Spratly Islands, which straddle key shipping lanes and are thought to contain rich energy reserves.

That row came days after an easing in a months-long standoff between China and the Philippines, but shows the persistent cycle of territorial frictions triggered by what some see as China's growing assertiveness in the area.

The South China Sea is potentially the biggest flashpoint for confrontation in Asia, and tensions have risen since the United States adopted a policy last year to reinforce its influence in the region.

At stake is control over what are believed to be significant reserves of oil and gas.

CNOOC, China's offshore oil specialist, said on its website last weekend that it would invite foreign partners to explore jointly and develop nine blocks in the western part of the South China Sea this year.

On Tuesday, Vietnam said CNOOC's plan was "illegal" and the blocks encroached on Vietnamese territorial waters.

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At a regular briefing on Wednesday, China's Foreign Ministry spokesman, Hong Lei, insisted that the tenders were in accord with Chinese and international law and urged Vietnam not to escalate the dispute.

(Editing by Jeremy Laurence and Robert Birsel)