HANOI—Vietnam accused a Chinese energy company of operating in its waters illegally, potentially ratcheting up tensions further between the two countries.

On Saturday, China's Maritime Safety Administration disclosed the location of China National Offshore Oil Corp.'s oil rig. The area is in part of the South China Sea that Vietnam claims as its "exclusive economic zone," said Vietnamese Foreign Ministry spokesman Le Hai Binh.

"All activities by a foreign entity in Vietnamese waters without Vietnam's consent are illegal and invalid, and Vietnam strongly protests [such activities]," Mr. Binh said in a statement posted on the government's website late Sunday. The area the Cnooc rig is operating in is "only 120 nautical miles from Vietnam's shore," the statement added.

At a daily press briefing on Monday, China Foreign Ministry spokeswoman Hua Chunying said the rig was in Chinese waters. "As we understand it, the Chinese Maritime Safety Administration issued a shipping notice on May 3 regarding the work of China's 981 platform. The relevant work is located completely within the area belonging to China's Xisha Islands," Ms. Hua said.

Cnooc didn't immediately reply to a request to comment on the situation.

State-owned Vietnam Oil & Gas Group, or PetroVietnam, Vietnam's biggest energy company, on Sunday sent a letter to Cnooc executives to protest the location of the oil rig, according to the government statement.

Vietnam and several other countries in the Asian-Pacific region have been embroiled in long-standing territorial disputes with China over parts of the South China Sea.

Last month, Vietnam officially launched a fisheries surveillance force that it said will monitor local fishermen and work to protect the country's territorial waters. This followed China's move to enact a new regulation in January requiring foreign fishermen to obtain Beijing's consent before operating in parts of the South China Sea that it claims, including the waters near the Paracel islands.

In 2012, the Vietnamese government objected to Cnooc's decision to invite bids for a new batch of oil exploration blocks, some of which were within the 200-mile limit that Vietnam claims as its exclusive economic zone. While Cnooc regularly offers blocks for foreign investment in the sea, the move marked its most significant offer in disputed waters. "Large-scale deep-water rigs are our mobile national territory and a strategic weapon," Cnooc Chairman Wang Yilin said at the time.

PetroVietnam urged China to cancel bidding for the areas it identified as being in Vietnamese waters, calling on foreign firms not to participate and noting that Oil & Natural Gas Corp. , Gazprom OAO and Exxon Mobil Corp. had been operating under licenses issued by Vietnam in some of those areas for many years.