President Donald Trump did not direct former national security adviser Michael Flynn to talk to the Russian ambassador about sanctions imposed on Moscow by the Obama administration, but says he would have.

Speaking at a White House news conference on Thursday, Trump said Flynn was "doing his job and I agreed with him".

"I would have directed him if he wasn't doing it," Trump added.

Trump asked for the resignation of Flynn because he was not happy with the way he had communicated information to Vice President Mike Pence.

"I don't think he did anything wrong. If anything, he did something right" in talking to Russian officials, Trump told a news conference on Thursday.

"The thing is, he didn't tell our vice president properly and then he said he didn't remember."