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Chinese fishing boats have returned to a lagoon in a disputed South China Sea shoal despite an agreement to clear the area of all vessels, dashing hopes of an early resolution of a territorial rift with the Philippines, officials in Manila said Wednesday.

Meanwhile, Vietnam protested a Chinese state oil company's invitation for bids for energy development in different area of the South China Sea, adding to concern that tension in the disputed waters could escalate.

Various longstanding disputes involving China, the Philippines, Vietnam, Taiwan, Malaysia and Brunei straddle busy sea lanes that are believed to be rich in oil and gas deposits. Many fear the disputes could spark a violent conflict.

The current standoff between China and the Philippines in the Scarborough Shoal off the northwestern Philippines began in April when the Philippines accused Chinese fishermen of poaching in its exclusive economic zone, including the shoal. During the tensions, both sides have sent government ships to the area.

A recent agreement saw both countries withdraw vessels, but the Philippines Department of Foreign Affairs spokesman, Raul Hernandez, said six Chinese fishing boats and 17 smaller dinghies were spotted by a Philippine plane inside the lagoon on Monday afternoon. He said five Chinese government ships were sighted outside the lagoon in the vicinity of the shoal.

Hernandez urged China to abide by its commitment in talks aimed at diffusing the rift.

"It is important for parties in negotiations and discussion on any issue to always act in good faith," he told reporters.

The Chinese Embassy in Manila did not immediately reply to a request for comment.

The United States, which says it has a national interest in the maintenance of peace and stability in the South China Sea, urged calm on both sides.

"It's in the interest of the United States and frankly, we believe, all the players, to see a de-escalation and calming process play out," Campbell told a seminar at a Washington think tank. He said he was heartened that Manila and Beijing were discussing the issue, and expressed hope for progress in resolving the standoff before an Asian security forum to be held in Cambodia next month where nations will be discussing how to manage the South China Sea disputes.

He warned that the disputes are long-standing and have stirred up nationalist sentiments. He said it was vital to deal with them with "extreme delicacy."

In Hanoi, Vietnam protested a weekend announcement by the China National Offshore Oil Corp., or CNOOC, that it was opening nine oil and gas lots for international bidders, in areas overlapping with existing Vietnamese exploration blocks.

Vietnam Foreign Ministry spokesman Luong Thanh Nghi said the lots being offered by China lie entirely within Vietnam's 200-nautical mile exclusive economic zone and continental shelf. He said in a statement posted Tuesday on the ministry's website that China's move was illegal and the bidding should cease immediately.

PetroVietnam President and CEO Do Van Hau told a news briefing in Hanoi on Wednesday that the Vietnam-owned oil and gas company would go ahead with exploration contracts it signed with foreign oil companies in offshore areas now being offered by the Chinese company to investors.

He urged foreign companies to ignore the Chinese offer.

The exploration areas CNOOC offered to international bidders overlap with the exploration contracts PetroVietnam signed with ExxonMobil, Russia's Gazprom, India's ONGC, and PVEP, a PetroVietnam affiliate, he said, adding the foreign companies have been conducting exploration in the area for many years.

Bonnie Glaser, a China expert at the Center for Strategic and International Studies in Washington, said it appeared to be the first time since 1992 China has offered blocks for exploration inside disputed waters. She said the move would trigger concern in the U.S. and the region, and did not bode well for the China and Vietnam managing their competing claims.

Glaser said major U.S. companies would think twice about participating in the bidding because of the high potential for some kind of conflict breaking out.

In Beijing, Chinese Foreign Ministry spokesman Hong Lei told a news conference Wednesday that it was natural for the state oil company to seek bids to explore the areas. Hong said China hopes Vietnam "will not further complicate and aggravate the dispute."

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Associated Press writer Tran Van Minh in Hanoi, Vietnam, and Matthew Pennington in Washington contributed to this report.