The U.S. embassy in London has issued a security alert for American citizens in the U.K. following a suspected terror plot near an RAF airbase used by the U.S. military, which led to five arrests on Sunday.

“U.S. citizens are encouraged to exercise caution and be mindful of personal security,” read the notice responding to the incident near RAF Fairford. “U.S.-associated facilities, gatherings, and organizations may be at heightened risk. U.S. citizens are advised to follow instructions of local authorities for their personal safety and security.”

The alert reminded Americans that the U.K. in April raised its national terrorism threat level to “severe” from “substantial,” a decision made after an antisemitic attack in London was declared a terrorist incident by police. The “severe” level, according to the Metropolitan Police, means “an attack is highly likely.” The warning came hours after British police arrested five men on suspicion of explosives and terrorism offenses after they were detained near RAF Fairford in southwest England, an airbase used by the U.S. to strike Iran, in the early hours of Sunday. Officers made the arrests after receiving a tip-off that three "suspicious vehicles" appeared to be heading toward the direction of the base, which Iran’s Islamic Revolutionary Guard Corps (IRGC) previously said could be a target on account of its use by the U.S. military.

The U.K.’s Counter Terrorism Policing (CTP) confirmed in a statement to TIME Monday that the five arrestees were British nationals in their 20s who live in London.

“Specially trained CTP investigators have been examining the scene overnight and have been carrying out a range of activity, which have led to multiple new lines of enquiry,” read the statement.

British Defense Secretary Wes Streeting told BBC Radio 4’s Today programme early Monday that it is too soon to speculate over the motives during a live investigation. Questioned about a potential Iran link, he said: "We know that Iran and their proxies wish to do us harm. We know that they hate our democracy, and we know that we regularly have to thwart Iranian attempts to threaten either our interests or indeed our allies." Similarly, CTP’s deputy assistant commissioner Vicki Evans said she was “very aware of the levels of speculation about the motivation behind this incident, and the geopolitical questions that are being raised,” but stressed more time is needed to work through the case. What we know about the arrests at RAF Fairford Gloucestershire Police, the local authority responsible for the area that includes the airbase, received a call from a concerned resident at 12:45 a.m. local time on Sunday, regarding three suspicious vehicles traveling toward the airbase.

The local resident, a farmer, told police they had found the road blocked by three white vans. “I saw a large group of hooded and masked men. When they saw me, they ran ahead of me… I then realised that something highly unusual was taking place,” she later told the BBC. Armed officers responded to the call and five men were arrested on suspicion of committing offences under the Explosives Act, as well as section 5 of the Terrorism Act.

Armed officers responded to the call and five men were arrested on suspicion of committing offences under the Explosives Act, as well as section 5 of the Terrorism Act.

Gloucestershire Police evacuated residents from around 85 nearby households and put a cordon in place while the British Army’s specialist explosive ordnance disposal team examined the vehicles. The incident is now being handled by CTP.

While the local resident who alerted police has been hailed for their actions, Streeting has indicated the person who made the tip-off only had a “partial picture” of the matter and appeared to suggest U.K. security forces had been aware of a suspected plot involving RAF Fairford.

U.S. President Donald Trump on Sunday claimed the suspects had been “under view for a long time.” "Working with Britain, it was an amazing job. We had them under investigation. They were looking to do big damage to our fort, and working with the British worked out great… and we got them,” Trump told reporters. When asked if Trump had been correct, Streeting did not reject the assertion.

“Our security services and our police, they don’t ask for praise, but I think it was a proud moment yesterday to see the President of the United States recognize the extraordinary work that our police and our security and intelligence services do every day throughout the year and throughout the world to keep us safe,” he said. “That’s all I want to say this morning on that.” TIME has asked the Ministry of Defense and Downing Street whether the British authorities were already investigating a potential plot.

“Our security services and our police, they don’t ask for praise, but I think it was a proud moment yesterday to see the President of the United States recognize the extraordinary work that our police and our security and intelligence services do every day throughout the year and throughout the world to keep us safe,” he said. “That’s all I want to say this morning on that.” TIME has asked the Ministry of Defense and Downing Street whether the British authorities were already investigating a potential plot.

Speaking at the Labour Party conference later Monday morning, Streeting celebrated the U.K. and U.S. alliance as “the closest of any two nations on Earth” and vowed the two countries “will always stand together against those who hate our freedom and those who wish us harm.”