BEIJING (Reuters) - China has complained to Japan about comments by a top Japanese diplomat that Taiwan's political status was up in the air, the Chinese Foreign Ministry said on Tuesday, testing ties still scarred by World War Two.

Taiwan activists protested at the de facto Japanese embassy in Taipei on Monday, three days after top Japanese envoy Saito Masaki told a university forum that the island's status had "not been determined", angering the Taiwanese government.

China sees self-ruled Taiwan as a breakaway province, to be united with the mainland eventually, and by force if necessary - its "one China" policy. Taiwan, which refers to itself as the "Republic of China", considers itself sovereign.

"We express our strong dissatisfaction (with the comments)," Chinese Foreign Ministry spokesman Ma Zhaoxu said in a statement on its website (www.mfa.gov.cn). "The Chinese government has already made solemn representations with the Japanese side."

Japan recognises China over Taiwan and thus cannot have an official embassy on the island. Only 23 countries, mostly small and impoverished, recognise Taiwan over China.

Japan maintains strong informal ties with the island it ruled for 50 years until 1945.

Saito said he was stating only personal views, and the de facto embassy said it took no official position on Taiwan's
"legal status".

Chinese spokesman Ma repeated Beijing's official position that Taiwan is an inseparable part of China.

"Any scheme to try to create discussion on Taiwan's status is a provocation of China's core interests and cannot be accepted by the Chinese government or its people," he added.

Ties between China and Japan are often testy due to Beijing's insistance that Tokyo has never properly atoned for its brutal occupation of large parts of China during World War Two.