China's President Xi Jinping told his U.S. counterpart Joe Biden that the issues of Taiwan, democracy, human rights and rights to development are "red lines" for China and not to be challenged, the official state media Xinhua said on Sunday.

Without mentioning President-elect Donald Trump's name, Chinese President Xi Jinping appeared to signal his concern that Trump's protectionist rhetoric on the campaign trail could send the U.S.-China relationship into another valley.

Chinese leader Xi Jinping has outlined his "four red lines" to US President Joe Biden. The hot-button issues concern democracy and China's development. Xi's comments came two months ahead of President-elect Donald Trump's inauguration in January.