BEIJING (Reuters) - China's Foreign Ministry urged the Philippines on Friday (Jan 22) to exercise "restraint" over "relevant parties", after Filipino protesters said they planned a second trip to contested islands in the South China Sea.

The Filipino protesters plan in April to spend a month on all the islands in the Spratlys that are occupied by the Philippines.

In December, about 50 Filipino protesters, mostly students, made a three-day boat trip to Thitu, one of nine outcrops or islands occupied by the Philippines.

Chinese Foreign Ministry spokesman Hong Lei repeated that China had "indisputable sovereignty" over the Spratlys.

"The Philippines' occupation and illegal seizure of the islands is against the law and without effect," Hong told a daily briefing in Beijing. "The Philippines should increase restraint on itself and relevant parties, and should not complicate matters," he said.

China claims almost the whole of the energy-rich South China Sea, which is also staked by Brunei, Malaysia, the Philippines, Taiwan and Vietnam, and through which about US$5 trillion (S$7.14 trillion) worth of ship-borne trade passes each year.