Factbox-What is known about arrests at US airbase in Britain

LONDON, Sept 28 (Reuters) - British police have arrested five men near an airbase used by the US for strikes on Iran.

Below is what is known so far about the incident at RAF Fairford:

SECURITY STEPPED UP RECENTLY

A local councillor, Tristan Wilkinson, told Reuters there had been a specific threat about six weeks ago from the Iranians against the base in western England, leading to increased surveillance in the area.

The British government, which has authorised the US to use bombers based atFairford to strike Iranian missile sites attacking ships in the Strait of Hormuz, said Prime Minister Andy Burnham was being updated on the incident.

Britain said in July its armed forces were ​ready to protect the country from any attack after Iran's Revolutionary Guards warned it not to allow US bombers to ‌fly from Fairford, saying any base used to launch attacks would be a legitimate target.

POLICE CALLED EARLY ON SUNDAY

Police said they received a call at 12:45 a.m. on Sunday relating to three suspicious vehicles that appeared to be travelling towards the air base.

Armed officers from Gloucestershire Police responded and arrested five men in the Whelford area, about a mile from the base. The five men were initially arrested on suspicion of offences under the explosives act and later further arrested on suspicion of preparation of a terrorist act. Counter-terrorism police are now leading the investigation.

VILLAGE EVACUATED

About 85 households were evacuated from properties near to where the three vehicles were found. A 400-metre cordon remains in place around them and the army's explosive ordnance disposal unit has conducted examinations.

Police said the public should expect to see an emergency services presence in the area for some time.

ARRESTED MEN APPEAR PICTURED

A photograph circulating in British media shows five men, their chests bare, being detained and handcuffed by armed police.

LOCAL FARMER ALERTED POLICE

A local farmer said she had tipped off police after she saw three large white vans parked near the base when she returned from a party around 1a.m. on Sunday morning, and witnessed a group of "hooded and masked men" running into the fields and towards Fairford.

The farmer, who has not been named in media reports, said she was worried because of the threat of an attack on the base.

British defence minister Wes Streeting told BBC TV that he was grateful to the witness but she only had a "partial picture".

US President Donald Trump said on Sunday that the group had been under investigation. "We had them under view for a long time, and we got them," he said, adding that the suspects intended "big damage" to the base.

(Reporting by Kate Holton, additional reporting by Sarah Young, editing by Andrew Cawthorne)