A dual UK-Iranian national arrested over a suspected terror plot at the US-run RAF Fairford air base has been released on bail, British counterterrorism police said on Saturday.

The 25-year-old man was arrested on Thursday in the Borough of Westminster, London, on suspicion of preparing terrorist acts under Section 5 of the Terrorism Act 2006. He is the sixth person arrested and bailed in the investigation.

Five other men, all British nationals in their mid-20s from London, were arrested near the base on September 27 and released on bail the next day. Police found three suspicious vans near the Gloucestershire base containing a quantity of petrol, but no improvised explosive devices.

Prime Minister Andy Burnham has said there are strong indications of Iranian involvement in the incident. Iran has denied any role.

Vicki Evans, senior national coordinator for Counter Terrorism Policing, called the inquiry hugely complex. She said specialist teams were examining all possible angles, including possible foreign state involvement. The investigation continues.