Taipei's maverick mayor Ko Wen-je, one of Taiwan's most senior and popular politicians, sparked heavy criticism Sunday after saying China should recognise the island as an independent democracy.

Taiwan split from the mainland in 1949 after a civil war but China still considers the island part of its territory.

Fears over warming ties with China have sparked mass protests in Taiwan and led to the trouncing of the ruling Beijing-friendly Kuomintang (KMT) party at local polls in November.

But politicians still tend to tread a diplomatic line, recognising that Beijing is an essential trading partner.

Ko broke with that etiquette in an interview with the US-based magazine Foreign Policy.

"Cooperation is more important than reunification. If reunification is achieved without cooperation, it will be meaningless," he said.

"We have to convince mainland China that a free and democratic Taiwan is more in China's interest than reunification."

Ko questioned the mainland's "one China" policy.

"What is one China? You have to tell me what one China looks like. If a girl is to marry into another family, you have to tell her what that family is like," he said.

Taipei mayors have frequently gone on to become president and while Ko has said he will not stand in the 2016 leadership vote, he is one of the island's most high-profile political figures.

"As Taipei mayor, Ko should not have placed his own political belief above the interests of Taipei citizens," KMT party spokesman Charles Chen said Sunday, adding that he could jeopardise fledging official links.

Ko should "take some freshman classes... so as to understand the definition of 'a nation'," said Liu Guoshen, director of the Xiamen University Taiwan Research Institute, quoted in the pro-Beijing China Times.

In the Foreign Policy interview Ko also said that Vietnamese culture was "superior" to China's, thanks to colonisation -- remarks which even drew anger from the liberal press and the China-sceptic Democratic Progressive Party (DPP), which backed Ko in the mayoral race.

"There's not any country in the world who would like to be colonised," said DPP legislator Huang Wei-che.

Though his public approval ratings are high, Ko is seen by critics as a loose cannon -- last week he left a British minister red-faced after saying he would sell a watch she gave him for scrap, as the gift of a timepiece is seen as bad luck locally.