BEIJING - China on Tuesday reaffirmed its indisputable sovereignty over the Nansha Islands and its adjacent waters and protested against the Philippines' plan to explore for oil and natural gas in the area of the South China Sea.

Foreign Ministry spokesman Hong Lei made the remarks at a regular news briefing when responding Philippine Secretary of Energy Jose Almendras' statement on inviting foreign investors to explore two oil and gas blocks under China's jurisdiction.

"It is unlawful for any country or company to explore oil and gas in sea areas under Chinese jurisdiction without the permission of the Chinese government," Hong said.

The Philippine Department of Energy is proceeding with plans to offer 15 offshore territories for oil and gas exploration, including two areas near Palawan province that are part of China's sovereign territory.

China protested the plans to invite foreign companies to explore in the two areas last year, saying they are part of the South China Sea region in waters where it has "historic titles" and "sovereign rights and jurisdiction".

However, Philippine officials continue to maintain that the two areas in question - Area 3 and Area 4 in the licensing round - are not part of the disputed area in the South China Sea and are well within the country's territory, The Daily Tribune reported.

Almendras said on Monday that the government would start awarding exploration contracts to some companies next month, but did not say whether the permits would include Area 3 and Area 4.

Companies are allowed to submit pre-qualification documents until Wednesday.

The Department of Energy will let the Department of Foreign Affairs deal with diplomatic issues, according to the Philippine Star.

"I am sure it will have diplomatic complications, which I am sure the DFA will be able to handle," Almendras said.

Vietnam, the Philippines, Malaysia and Brunei claim sovereignty over some areas in the South China Sea, which are rich in natural resources.

Hong urged relevant countries to adhere to the spirit of the Declaration on the Conduct of Parties in the South China Sea and refrain from taking moves that will complicate or aggravate the dispute, and make due efforts for the peace and stability of the South China Sea.

The huge potential interest in energy exploration is driving the Philippines to enlarge its presence in the area, said Zhang Tuosheng, a researcher at the China Foundation for International and Strategic Studies.

"Its economic interference is against the spirit of Declaration on the Conduct of Parties in the South China Sea - changing the status quo in the disputed area," he said.

Wang Yingfan, former Chinese ambassador to the Philippines, said last week that China and the Philippines should jointly explore and develop some areas in the South China Sea.

During a meeting with Philippine Foreign Affairs Secretary Albert del Rosario, Wang reiterated China's position that the Philippines should not allow a third party, such as the United States, to help resolve the problem, adding that "with goodwill and hard work, we could find a way out that's agreeable and acceptable to both sides".