TAIPEI: Chinese President Xi Jinping warned that relations between Beijing and Taipei were "grim" on Sunday, urging the island's main opposition party to help seek "unification of the country."

China views self-ruled democratic Taiwan as part of its territory and vows to retake it one day, by force if necessary.

Xi has become the most bellicose leader since Mao Zedong, describing the seizure of the island as "inevitable."

In a congratulatory letter to Eric Chu -- the newly elected leader of the Beijing-friendly Kuomintang (KMT) party -- Xi said the Chinese Communist Party and the KMT should collaborate under a "shared political basis."

"In the past our two parties insisted on '1992 consensus' and opposing 'Taiwan independence' ... to promote peaceful developments in cross- strait relations," Xi said in the letter released by the KMT.

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"At present the situation in the Taiwan Strait is complex and grim," he said, urging the parties to jointly seek peace and "the unification of the country."

Ties between Taiwan and China improved markedly under former president Ma Ying-jeou of the KMT between 2008 and 2016, culminating with a landmark meeting between Xi and him in Singapore in 2015.

The KMT has side-stepped frictions with China by accepting the so-called 1992 consensus -- a tacit agreement that there is only "one China" without specifying whether Beijing or Taipei is its rightful representative.

In response, Chu said in a letter to Xi that the two sides should "seek common ground and respect their differences" to promote peace and stability in the Taiwan Strait.

Beijing has stepped up military, diplomatic and economic pressure on Taiwan since the 2016 election of President Tsai Ing-wen, who views the island as a sovereign nation and not part of "one China."

Last year, Chinese military jets made a record 380 incursions into Taiwan's defence zone, with some analysts warning that tensions between the two sides were at their highest since the mid-1990s.

On Thursday, China flew 24 warplanes including two nuclear-capable bombers into Taiwan's air defence zone, the biggest incursion in weeks, after voicing its opposition to Taipei joining a major trans-Pacific trade deal.