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 President Xi Jinping warned the United States on Saturday not to cross a "red line" in support for Taiwan, but told his counterpart Joe Biden that Beijing was willing to work with the incoming administration of Donald Trump.