US President Donald Trump's counter-terrorism adviser compared Islamic State militants fleeing the Middle East to "cockroaches", vowing to "obliterate" the extremist group, the media reported.

"We're going to obliterate IS. We're going to wipe them off the face of the Earth," White House deputy assistant Sebastian Gorka told  Fox News on Saturday.

"But before we get all of them, what's going to happen? Some of them, like the cockroaches, when you switch the light on they're going to scurry. We want to stop them coming here."

Trump's executive order barring citizens of six Muslim-majority countries from entering the US is geared toward blocking militants travelling from the Middle East to America, Gorka said.

"So it's not the person flying from Los Angeles to New York that's the threat," he said.

"It's the IS recruit, who's travelling out of Mosul (Iraq), out of the Middle East, out of Syria."

Gorka's comments came after a terrorist attack in London on Wednesday committed by a 52-year-old UK-born man, The Hill magazine reported.

IS claimed responsibility for the attack, which killed five people and left at least 50 people injured.