International experts have warned foreign companies not to respond to China's recent invitation for bids to explore nine oil and gas blocks that lie in Vietnam's sovereign territory.

Vietnam News Agency Thursday quoted Carlyle Thayer of the University of New South Wales in Australia as saying that the invitation by the China National Offshore Oil Corporation (CNOOC) was an act of revenge by China against Vietnam for recently passing the Law on the Sea of Vietnam.

It was a political rather than a commercial act, he said.

According to Thayer, the blocks are located completely in Vietnam's exclusive economic zone, the 200-mile maritime area extending from the country's coastline.

Dr Bonnie Glasser, a China specialist at the US-based Center for Strategic and International Studies (CSIS), also warned that any foreign company planning to take part in the bidding must see the high risks and needs to think twice, the VNA report said.

The two experts were attending the second annual Maritime Security in the South China Sea Conference organized by the CSIS Southeast Asia Program on June 27-28 in the US capital Washington DC.

The event was attended by scholars and experts from several countries including Vietnam, China, and Japan, who discussed the East Sea issues as well as international laws and customs applicable in resolving disputes.

Earlier, on Tuesday, Vietnamese Foreign Ministry spokesman Luong Thanh Nghi condemned CNOOC's action, saying it was a serious violation of Vietnam's sovereignty.

The next day Do Van Hau, director general of the state-owned Vietnam Oil and Gas Group (PetroVietnam), which has been exploring blocks overlapping with the blocks eyed by the Chinese company, also expressed condemnation.

He called on foreign companies not to join the bidding, and stressed that his company's activities and plans would not be affected by the "illegal" bid invitation.