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.