British police have released on bail five men arrested near a Royal Air Force base used by the United States, authorities confirmed on Monday.

Washington asserted that a "foreign actor" was involved in the suspected bomb plot linked to the incident.

US President Donald Trump, speaking in Washington, said he was "surprised that they released them."

The release of the five men came as British Prime Minister Andy Burnham announced an emergency meeting of top officials to ensure they have "the full picture."

Police described the investigation as ongoing.