Continuing his battle with mainstream media outlets, US President Donald Trump slammed "fake news" and said journalists should not be allowed to publish stories based on anonymous sources.

"A few days ago I called the fake news the enemy of the people, and they are - they are the enemy of the people," USA Today quoted Trump as saying at the annual Conservative Political Action Conference here.

Trump claimed that "the fake news media doesn't tell the truth", adding that reporters should not be allowed to use anonymous sources, and "we're going to do something about it".

He also said major media outlets made up polls during the campaign to give the impression that he had no chance.

"Take a look at some of these polls. That were so bad, so inaccurate. And what that does is it creates a false narrative. It creates this narrative that it's just like we're not going to win," the President added.