Vietnam has decried China's plan to open up disputed areas of the South China Sea to oil exploration as an illegal move, ratcheting up tensions between the two Communist neighbors over maritime borders and natural resources.

The China National Offshore Oil Corp. (CNOOC) announced Saturday that it was seeking bids from foreign companies to search for oil within nine offshore blocks.

Vietnam's Foreign Ministry responded Tuesday, saying the blocks lie entirely within Vietnam's 200-mile exclusive economic zone.

This is absolutely not a disputed area, it added, calling CNOOC's move illegal and a serious violation of Vietnam's sovereignty.

Chinese Foreign Minister Hong Lei defended CNOOC's actions, saying that it was conducting normal business activity.

China and Vietnam have reached many agreements regarding the settlement of maritime disputes, Hong added. We hope Vietnam will respect these agreements and avoid taking any action that may complicate the matter.

China and Vietnam have had a longstanding dispute over territorial claims in the South China Sea, particularly over two archipelagoes known as the Paracel and Spratly islands, which are believed to be rich in natural gas and oil reserves.

Last week, Vietnam passed a law reaffirming its claims to the two island chains in a snub to China's proclaimed sovereignty and possibly serving as the provocation for CNOOC's move.

China has claimed the majority of the South China Sea, citing a centuries-old history of maritime exploration in the region.

Neighboring states the Philippines, Malaysia, Brunei and Taiwan (which China considers a renegade province) have also made competing claims to various parts of the sea.

Meanwhile, as an ally of the United States, Vietnam's refusal to allow Chinese exploration activities in this part of the South China Sea may be seen as yet another wrinkle in the alweays tense realtions between Washington and Beijing.

In July 1995, US President Bill Clinton announced the normalization of diplomatic relations between the U.S. and Vietnam, twenty years after the end of the Vietnam war.

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