US President Donald Trump has spoken out for the first time in person about the resignation of Mike Flynn as national security adviser.

Flynn resigned after a series of leaks from at least nine current and former senior officials in US intelligence and law enforcement revealed prior to Trump taking office, Flynn held calls with Russia's US ambassador and suggested sanctions could be lifted under the new administration.

Trump, speaking during a press conference with Israeli Prime Minister Benjamin Netanyahu said Flynn had been treated badly by 'fake media' and blamed Hillary Clinton's election loss for White House leaks.

"I think he's been treated very, very unfairly by the media — as I call it, the fake media, in many cases," Trump said in a press conference on Wednesday reported by The Hill.

"I think it's really a sad thing he was treated so badly."

The president also commented on the topic of leaked information – something that appears to be plaguing the current administration, with White House press secretary Sean Spicer vowing to tackle the problem.

"From intelligence, papers are being leaked, things are being leaked, it's criminal action, Trump said.

"It's a criminal act, and it's been going on for a long time before me, but now it's really going on.

"People are trying to cover up for a terrible loss that the Democrats had under Hillary Clinton," he added.