BEIJING, Jan. 22 -- China on Friday reaffirmed that any activity by the Philippines in China's territory the Philippines has occupied is illegal and invalid.

It is reported that a Philippine youth group called "Kalayaan Atin Ito" said on Thursday that they plan a second trip to the disputed islands in the South China Sea in April. About 50 group members made a trip to disputed islands in December last year.

"China has indisputable sovereignty over the Nansha Islands and their adjacent waters," Chinese Foreign Ministry spokesperson Hong Lei told a regular press briefing. "Any activity by the Philippines on some of the islands and reefs of China's Nansha Islands the Phillippines occupies is illegal and invalid."

The Philippines has the responsibility to keep relevant groups or members under control to avoid a more complicated situation, Hong said.