Chinese compatriots across the Taiwan Strait have expressed strong indignation and opposition against Japan's so-called "purchase" of the Diaoyu Islands, a mainland official said Wednesday.

Measures taken by each side of the Strait to safeguard the interests of the entire nation will be supported firmly by all the Chinese people, said Fan Liqing, spokeswoman of the State Council Taiwan Affairs Office, at a press conference.

"Brothers quarrelling at home join forces against attacks from outside," Fan quoted from ancient Chinese work of "Shi Jing," or the "Book of Odes."

"It's an unshakable duty and common responsibility for compatriots across the Strait to safeguard territorial sovereignty of the Diaoyu Island and its affiliated islets as well as the interests of the entire nation," she said.

The islands have been China's territory since ancient times. Cross-Strait compatriots have common cognition on this fact, Fan said, adding that they will never accept Japan's illegal encroachment on the Diaoyu Islands.

Moreover, in response to reporter's questions, Fan said China had indisputable sovereignty over the South China Sea islands and their adjacent waters.

People from both sides of the Strait are responsible for safeguarding the country's territorial sovereignty and the mainland was happy to see that companies and non-government institutions across the Strait are conducting constructive discussions on relevant issues, she said.