Taiwan's presidential frontrunner Tsai Ing-wen urged authorities in Hong Kong and Beijing Wednesday to explain the disappearance of five Hong Kong booksellers feared detained by mainland Chinese authorities.

The missing men all worked for the same Hong Kong-based publishing house known for books critical of the Chinese government, in a case that has sparked anger in the city.

Tsai's comments come the week before an election in which she is tipped to unseat the Beijing-friendly ruling Kuomintang (KMT) party, raising questions over cross-strait relations.

Tsai, who leads the Beijing-sceptic opposition Democratic Progressive Party (DPP), has said she wants to preserve the "status quo" with China if she becomes president -- but opponents say she would destabilise ties.

The DPP traditionally leans towards asserting independence, which has never formally been declared by Taiwan.

"(I) hope that Hong Kong and Beijing authorities... will give a clear explanation," Tsai said Wednesday at a campaign event in Taipei.

"At the same time, (they) should implement concrete measures to ensure freedom of speech in Hong Kong," she said.

"Taiwanese people care deeply about freedom of speech, because Taiwan after all went through a very difficult period to win our freedom of speech."

The island was ruled under martial law by KMT leader Chiang Kai-shek until 1987.

Large-scale protest movements flared in both Hong Kong and Taiwan in 2014, with protesters angry at growing Chinese influence.

Taiwan split from China in 1949 after a civil war on the mainland, but Beijing still considers it part of its territory awaiting unification -- by force if necessary.

Hong Kong is semi-autonomous after it was handed back to China by Britain in 1997 and enjoys freedoms unseen on the mainland, but there are increasing fears those are under threat.

The KMT's China-friendly policy has been a major source of criticism. But its presidential candidate Eric Chu also voiced concern Tuesday about the disappearances.

"The Hong Kong or Chinese authorities must provide a clear answer... as pursuit of democracy and freedom is a universal ideal," he said during a campaign event in southern Taiwan, according to the Apple Daily.

China's Foreign Minister Wang Yi did not reply directly when asked Tuesday whether China had detained the booksellers, but said policy towards Hong Kong remained unchanged.