The war between Donald Trump and CNN has taken a new turn with the government's challenge to a tie-up between AT&T and the cable news channel's parent firm Time Warner.

Even as his Justice Department moved to block the deal, the president has stepped up his attacks on the major news network, his most frequent target of "fake news" complaints.

Trump has been attacking CNN since the 2016 campaign, but the latest developments raise questions about whether he is using his power to choke off investment, or even force CNN to be sold to a partner friendlier to the White House.

As the atmosphere has deteriorated, CNN said it would boycott the White House Christmas party "in light of the president's continued attacks on freedom of the press and CNN."