Taipei (AFP) - A row over a teenage Taiwanese K-pop star forced to apologise for waving the island's flag was rapidly turning into an international incident Saturday, stealing the spotlight from Taiwan's presidential elections.

Leadership candidates running for office in Saturday's pivotal vote demanded answers from China and South Korea after the video apology from a tired and sombre Chou Tzu-yu, 16, went viral, with more than 2.6 million hits.

The controversy dominated news coverage in Taiwan on election day, with China ties a key issue for voters.

"There is only one China and the two sides are one," Chou said in the apology, posted on YouTube by South Korean agency JYP Entertainment.

"As a Chinese person my improper words and behaviour during my activities abroad hurt my company and the feelings of netizens across the strait," Chou added.

"I have decided to stop my activities in China for now to seriously reflect on myself."

Based in South Korea, Chou is a singer with all-girl band TWICE.

She waved Taiwan's official flag on a recent online broadcast, angering Chinese netizens, with suggestions she was pushing a pro-Taiwan independence agenda.

Taiwan has ruled itself since a split with the Chinese mainland in 1949 after a civil war, but has never formally declared independence.

Beijing sees it as part of its territory awaiting reunification, by force if necessary.

China's Taiwan Affairs Office blamed "political forces in Taiwan" for using Chou's case to "provoke conflict", according to a report from state news agency Xinhua, posted on the affairs office website.

"Compatriots on both sides of the strait should be on high alert," the report said, citing an unnamed TAO spokesman.

- 'Bullied by China' -

Both Taiwan's main leadership candidates defended Chou Saturday.

"A citizen of the Republic of China (Taiwan's official name) who shows her recognition for the country should not be suppressed and should not be forced to say the opposite of what she originally meant," said presidential frontrunner Tsai Ing-wen of the Beijing-wary Democratic Progressive Party.

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"Our hearts are with all friends who hold the flag and support the flag," said Eric Chu of the ruling Kuomintang (KMT), set to be punished in the polls for its Beijing-friendly stance.

A statement from the office of current President Ma Ying-jeou said he had asked officials to "clarify the roles of the Korean and Chinese sides in this incident and demand they seriously face it".

Ma later said waving the flag was not a sign of being pro-independence and it was unacceptable that Chou was forced to apologise.

There was also anger online in Taiwan.

"A 16-year-old girl is bullied by China. I sure will go voting today," said one post on Taiwan's Apple Daily newspaper website.

South Korean fans described the teen as a political victim and criticised her agency for kowtowing to China for money.

"This just shows how blinkered China is, using a child to score political points," said one post on South Korea's Naver news portal.

Analysts said the drama could boost election turnout and may help the DPP.

Media reports said a Chinese TV channel had cancelled an upcoming show featuring TWICE and that Chou had also been pulled from endorsing Chinese smartphone vendor Huawei in the South Korean market.

Last year, US singer-songwriter Katy Perry caused a stir in both Taiwan and China when she appeared on stage in Taipei draped in a Taiwanese flag, while donning a gold dress adorned with sunflowers -- a symbol of anti-China student protests in Taiwan in 2014.