China warned Taiwan that declaring independence would be a "dead end", state media said Wednesday, after the island's democratically elected president phoned Donald Trump in a precedent-breaking move.

Beijing's stance of opposing and blocking "Taiwan independence splittism" is "firm and unmovable", Taiwan Affairs Office spokesman An Fengshan said at a briefing Wednesday, according to state-owned China News.

"We have unshakable willpower, ample confidence, and sufficient capability," he said. "Facts will show those people that 'Taiwan independence' is a dead end."

The comments came after Trump, the US president-elect, shocked the diplomatic establishment and angered Beijing by speaking directly with Taiwanese leader Tsai Ing-wen after his election victory.

After fierce criticism of the move, he upped the ante, taking to Twitter to ask why he should not be allowed to speak to Tsai, then attacking Chinese foreign and economic policy.

On Sunday, the billionaire businessman went a step further and suggested he could jettison Washington's decades-old "One China policy" -- a compromise allowing the US to do business with both China and Taiwan while only recognising Beijing diplomatically.

The remarks were a step too far for Beijing, which had initially seemed resigned to taking a wait-and-see approach to the president-elect.

On Monday, Chinese foreign minister Wang Yi warned Trump that anyone who challenges Beijing's interests on the island will "lift a rock only to crush his own toes".

China sees Taiwan, which has not been ruled by Beijing for more than 60 years, as a rogue province awaiting reunification, by force if necessary.

Cross-strait relations have deteriorated since Tsai, the island's first female president, took office in May after her China-sceptic Democratic Progressive Party won a landslide victory.