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.