Alec Baldwin has returned to Saturday Night Live as President Donald Trump in a cold-opening designed to remark on his questionable sources.

Set at a military base under alien attack, which is a spoof of the movie Independence Day, Baldwin's Trump gets debriefed by an officer, played by SNL regular Kenan Thompson.

"Actually, I heard the aliens are already here," Baldwin's Trump says. "They've been hiding in this country for hundred of years. They are shape-shifters."

Thompson's character asks: "Where are you getting this information?"

"From a very reputable source: Infowars," Baldwin's Trump. "It's a radio show hosted by Alex Jones. You know he's legit because he's always taking off his shirt."

Jones, it should be noted, is a far-right conspiracy theorist, whose show Infowars has been labelled as a fake news website.

Baldwin has already signalled that his time impersonating the president is numbered, recently telling Extra TV: "The maliciousness of this White House has people very worried, which is why I'm not going to do it much longer, by the way, the impersonation. I don't know how much more people can take it, you know."

Baldwin last appeared on SNL a month ago (on February 11) and even then he took a back seat to Melissa McCarthy's spoof of Sean Spicer, who cold-opened the show.

SNL's guest host Scarlett Johansson this week portrayed first daughter Ivanka Trump, spoofing a commercial for Ivanka's fictitious perfume, 'Complicit', in an attempt to question her values too.

"A feminist. An advocate. A champion for women," says the voiceover as the camera then pans to three skeptical women, with the voiceover adding, "but, like, how?"