Taiwan's presidential candidates waded in to a row with China and South Korea Saturday after a teenage Taiwanese K-pop star was forced to apologise for waving the island's flag.

A video apology from a tired and sombre Chou Tzu-yu, 16, went viral hours after it was posted, with more than two million hits by Saturday morning.

The controversy dominated news coverage in Taiwan on the day the island votes for a new leader, with China ties a key issue.

"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 to seriously reflect on myself."
Based in South Korea, Chou is a singer with all-girl band TWICE.

She waved the island'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 a renegade province awaiting reunification, by force if necessary.

- 'Bullied by China' -

Both Taiwan's main leadership candidates defended Chou Saturday as they cast their votes.

"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.
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 of the KMT 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.

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. 

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 draped in a Taiwanese flag at a concert in Taipei, while donning a glittering gold dress adorned with sunflowers -- the symbol of anti-China student protests in Taiwan in 2014.

In 2013, Taiwanese singer Deserts Chang was forced to cancel a performance in China amid controversy about her display of a Taiwanese flag at a concert in Britain.