The British foreign secretary has warned Russia not to try to use Paul Whelan, the British–American national arrested on spying charges, as a diplomatic pawn.

Jeremy Hunt said the UK was “extremely worried” about Hunt and had offered consular assistance. It emerged on Friday that Whelan, detained a week ago in Moscow, also holds Irish and Canadian passports, adding more layers of complexity to a potentially fraught diplomatic incident.

It has already been suggested that Whelan is being held as leverage to negotiate the release of Maria Butina, a Russian who recently pleaded guilty in the US to conspiring to act as an unregistered agent for Moscow.

Butina signed a broad cooperation agreement with the US justice department. Prosecutors said she tried to build a back-channel between Kremlin officials and Republican operatives during the 2016 presidential campaign.

Hunt fuelled speculation that Russia was using Whelan as some form of bargaining chip by telling Sky News: “We don’t agree with individuals being used in diplomatic chess games.”

The US has so far been leading on the case, but Hunt said the UK was also offering consular assistance. It has also been reported that Whelan, born in Canada, but also holding an Irish passport, is seeking Irish consular assistance.

Whelan is being given legal assistance by a Russian lawyer, including a request for bail, but there is concern about some of the advice being provided.

The Russian Interfax agency reported on Thursday that Whelan was being formally charged with espionage. He was arrested a week ago by members of the Russian Federal Security Service (FSB), who accused him of being on a “spy mission”, and he has since been detained at Lefortovo prison in Moscow.

Asked whether Russia was detaining Whelan to use as leverage against the UK or Russia, Hunt said: “We are not ruling out any theories at all at this stage as to why this might have happened.”

He added: “Our position is very, very clear ... Individuals should not be used as pawns of diplomatic leverage [or be] used in diplomatic chess games. We need to see what those charges are against him to understand whether there is a case or not.”

He added: “We are extremely worried about Paul Whelan. We have offered consular assistance. The US are leading on this because he is a British and American citizen.”

Jon Huntsman, the US ambassador to Russia, met Whelan at the prison on Wednesday and talked to his family, the US state department said.

Whelan, 48, is the director of global security for the auto suppliers BorgWarner, and travelled to Moscow last month to attend the wedding of a friend, according to his family.

He is a former police officer and marine discharged in 2008 for bad conduct on charges relating to larceny. He faces up to 20 years in prison if convicted of espionage.

John Sipher, a veteran of the CIA’s national clandestine service who ran the agency’s Russia operations, said Whelan’s record and behaviour made him an unlikely spy.

“We used to say Moscow is the Yankee Stadium of espionage,” Sipher said. “They have the best counter-intelligence service in the world and we knew they would be laser-focused on our operations.

“We would only run the top, most critical cases and use people we can best protect. We don’t use Americans who don’t have diplomatic immunity.”

Anyone caught without immunity would end up as “trade bait” – used to exchange for Russian spies held in the US, Sipher said – a situation the CIA would try to avoid at all costs.

“And we don’t hire people who are court-martialled for theft. It makes no sense,” he said.

Mike Pompeo, the US secretary of state, said: “We’ve made clear to the Russians our expectation that we will learn more about the charges, come to understand what it is he’s been accused of and if the detention is not appropriate, we will demand his immediate return.”

A Russian news agency has alleged that Whelan received a USB drive that contained the names of people employed at a secret state organisation.

Citing a security service source, Rosbalt news agency said a Russian citizen gave Whelan the USB when he visited him in his room at the Metropol hotel in Moscow last Friday. It is alleged that Whelan is a regular visitor to Russia and befriended potential contacts on VK, or VKontakte, the Russian equivalent of Facebook.

On Friday Whelan’s brother David issued a statement on behalf of the family thanking Huntsman for the prison visit.

“Our focus remains on ensuring that Paul is safe, well treated, has a good lawyer, and is coming home. We urge the US Congress and the state department to help on Paul’s behalf to secure his release and return him home soon,” the statement said.

Whelan had a social media account on VK for more than 10 years. He has about 60 VK friends, many of whom are linked to academies run by Russia’s navy or defence ministry, as well as the country’s civil aviation authority. Almost all his VK contacts are men.

In online conversations with the Guardian, Whelan’s VK friends said they doubted the allegations of espionage against him. “I believe this is a mistake,” one said, speaking on condition of anonymity. He also said he had shown Whelan around his home town – a provincial Russian city – about 10 years ago. “He didn’t ask any questions that seemed suspicious.”

Another VK friend said they had discussed everyday issues during online chats. “I asked him about his country,” the person said. “For the whole time I was in touch with Paul, he behaved correctly and respectably. He made a good impression.” His online friends said he seemed genuinely interested in finding out about Russia and its culture.

Although he has been visiting Russia since at least 2008, Whelan’s Russian is believed to be poor. His VK friends said he spoke English with them, or used Google Translate. He posted a number of brief congratulatory messages on Russian public holidays, such as World War Two Victory Day and Defenders of the Fatherland Day.

On 9 November 2016, the day after Donald Trump was elected US president, Whelan wrote a Russian-language post on VK that read: “Forward, President Trump!”