Hsiao Bi-khim, a former envoy to Washington who is under Chinese sanctions, is the running mate to presidential poll frontrunner Vice President Lai Ching-te.

Beijing has blasted the Democratic Progressive Party's (DPP) ticket as an "independence duo".

China regards self-ruled Taiwan as its territory, and in recent years has ramped up military and political pressure against the island -- currently governed under an administration that refuses to acknowledge Beijing's claim.

Hsiao -- seen as a deft diplomat responsible for strengthening Taipei's ties with Washington -- said it was important to "expand our support among the American public".

"A rock-solid partnership with the United States is critically important right now," she told reporters at a press conference Thursday.

"Taiwan cannot become an issue of partisan difference in America."

Like much of the rest of the world, Washington does not diplomatically recognise Taiwan. But it remains the island's most important ally.

China has long vowed to seize Taiwan for "reunification", by force if necessary -- a line that the opposition Kuomintang (KMT) party has echoed by painting the election as a choice between peace and war.

But Hsiao said "war is not an option".

Reiterating her party's commitment to maintaining the status quo, she said "it's also important that the international community... make clear to our counterparts across the Taiwan Strait that dialogue is the only way to resolve differences".

- Opposition alliance breakdown -

From Beijing to Washington, observers are closely watching Taiwan's January 13 election for its potential effect on the island's future relations with China -- which has severed high-level contact with President Tsai Ing-wen's administration.

The KMT's candidate Hou Yu-ih and third-party candidate Ko Wen-je, from the less established Taiwan People's Party (TPP), have trailed behind the DPP in opinion polls. Both have pledged warmer ties with China.

The two parties last week agreed to a unity ticket for a better chance against the DPP's Lai, but negotiations broke down over who should be the presidential candidate.

On Thursday, the two candidates arrived at a Taipei hotel in a meeting brokered by manufacturing tycoon Terry Gou, who had also announced a run for president.

KMT heavyweight and former president Ma Ying-jeou was also present as "a witness", Ma said, as well as party head Eric Chu.

The meeting -- held in front of Taiwan's local and international press -- descended into barbs between Ko and Hou, with no clear answers on how to resolve their differences.

"All I want is to pick a pair that is most likely to win," Ko said.

The deadline to register presidential candidates is Friday.