PARIS (AFP) - France's top diplomat on Wednesday (Dec 14) slammed the bullish tone taken by US President-elect Donald Trump with key trade ally Beijing in threatening to upend Washington's One China policy.

"Watch out for China... It's a big country. You can have disagreements with China but you don't talk like that to a partner," remarked French Foreign Minister Jean-Marc Ayrault on France 2 television.

He was responding to apparently off-the-cuff remarks by Trump in an interview with Fox News Sunday in which he questioned whether Washington should continue its "One China policy" unless Beijing makes concessions on trade and other issues.

"When China feels like it is being questioned over the unity of China, it's not necessarily very smart," he said.

US diplomatic tradition has long held to this policy, which does not recognise self-administered Taiwan as a sovereign state but sees Beijing as the sole representative.

It is essentially a diplomatic compromise allowing the US to do business with both China and Taiwan while only recognising Beijing diplomatically.

"I don't want China dictating to me," Trump said, defending an earlier phone conversation with the president of Taiwan.

"I don't know why we have to be bound by a One China policy unless we make a deal with China having to do with other things, including trade."

Beijing swiftly hit back, saying it was "seriously concerned" by Trump's suggestion he could drop the longstanding policy, saying it could have a big impact on bilateral ties.