Washington DC - President Donald Trump said Monday that British police should not have released on bail five men who were arrested near an airbase used by the US over a suspected bomb plot.

Trump added that the case "might be" linked to Iran but declined to give details.

"I'm surprised that they released them. I wouldn't have done that," Trump told reporters in the Oval Office, while adding that British authorities had worked "very closely" with the US.

"We know everything about them. We would not have released them."

Donald Trump Trump fuels bipartisan outrage with new round of "illegal" TV ads funded by taxpayers

The five men were arrested early Sunday near the Royal Air Force (RAF) Fairford base in southwest England, from which US bombers have taken off to target Iran since March.

British counter-terrorism police announced on Monday that the five men – all London-based Britons in their mid-20s – had been freed on bail, and that officers were probing whether a "foreign state" could be involved.

Asked whether the case was linked to Iran, Trump replied: "It might be."

Pressed to give details, Trump said: "I'd rather not. I mean I know the answer, I could be very specific, but I'd rather not say. I would not have released them."

Iran has denied involvement in any plot to target the base.