Russia has claimed it caught a British citizen who is being held on spying charges “red-handed”.

Paul Whelan was imprisoned as he was carrying out illegal activities in his hotel room in Moscow, Russian foreign minister Sergei Lavrov alleged.

Mr Whelan, a former US marine who also holds American, Canadian and Irish passports, was detained by Russia’s Federal Security Service (FSB) on 28 December.

His family have asserted he is innocent and said he was in Moscow to attend wedding celebrations.

“He was caught red-handed,” Mr Lavrov said during a news conference on Wednesday. ”He was detained at the moment he was carrying out specific illegal actions in his hotel.”

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Mr Lavrov said the investigation into Mr Whelan continued and his detention was not motivated by any desire to try to exchange him for Russian citizens held in the US.

He also said Mr Whelan’s brother was in Moscow and had been briefed about prison conditions, but David Whelan told Reuters on Tuesday such reports were “false”.

“Neither his parents nor his siblings are flying to Russia, and we have no plans to fly to Russia,” he said.

The FSB has opened a criminal case against Paul Whelan but offered no details of his alleged activities. An espionage conviction in Russia carries a prison sentence of 10 to 20 years.

Russian online news site Rosbalt.ru earlier this month cited an unnamed Russian intelligence source saying Mr Whelan had been detained five minutes after receiving a thumb drive containing a list of all the employees of a secret Russian state agency.

The same source was quoted as saying Mr Whelan had been spying for 10 years using the internet to identify targets from whom he could obtain information and the employee list he was caught with had long been of interest to US spies.

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Intelligence experts have suggested Moscow may have arrested Mr Whelan in retaliation for the US arresting a Russian national, Maria Butina, who admitted last month to US prosecutors she had tried to infiltrate American conservative groups as an agent for the Kremlin.

Mr Whelan’s arrest has further soured relations between Moscow and Washington, which have worsened over Russia’s 2014 annexation of Crimea, and subsequent economic sanctions and accusations of meddling in the US presidential election.