Beijing made a third solemn representation to Manila and warned that it is hard to be optimistic about a territorial impasse over an island. Authorities say they have prepared for any escalation of the situation by Manila.

Vice-Foreign Minister Fu Ying summoned Alex Chua, charge d'affaires of the Philippine Embassy in Beijing, on Monday. It was the third summons following previous ones on April 15 and 18.

"I called to see you twice last month, asking the Philippine side to calm down and refrain from taking action that would escalate or complicate the situation," Fu said.

Despite the two summons last month, "the Philippine side has not realized that it is making serious mistakes and instead is stepping up efforts to escalate tensions", she added.

In the wake of the incident, the Philippine side continued to send government vessels to the lagoon of Huangyan Island and repeatedly made erroneous remarks that mislead the public in the Philippines and the international community and played up public feelings, said Fu.

Manila's moves are "severely damaging the atmosphere of the bilateral relations between China and the Philippines", Fu said. "Therefore it is hard for us to be optimistic about the situation."

Beijing says Huangyan Island has been an integral part of China's territory for centuries and expects Manila not to "misjudge the situation and not escalate tensions without considering the consequences".

Beijing urged Manila to withdraw all Philippine vessels in the island waters and warned Manila not to disturb operations by Chinese fishing boats as well as those by the Chinese government's law enforcement vessels, Fu said.

On April 10, a Philippine warship entered the island's territorial waters, dispatched personnel to harass Chinese fishing boats and attempted to detain Chinese fishermen.

Beijing said the move infringed on China's sovereignty. Two Chinese patrol ships in the area later came to the fishermen's rescue, and the warship left.

But the impasse continued as Philippine vessels were reported to be still in China's territorial waters on Monday.

Given recent challenges by the Philippines, China's patrol ships will remain alert in the waters of Huangyan Island, Fu said.

Chinese fishery administration vessels will also supervise the fishing boats according to the law and provide services to ensure Chinese fishermen can perform their job in their traditional fishing areas without interference, Fu added.

Fu Ying stated that the Chinese side will keep to its position to seek a diplomatic solution to the current situation and China again urged the Philippines to seriously respond to Beijing's concerns and return to trying to resolve the situation diplomatically.