Moscow (CNN) It's too early to discuss a possible "prisoner swap" involving detained American Paul Whelan, Russia's deputy foreign minister said Saturday, according to a Russian state news agency.

Whelan, a citizen of the United States, the United Kingdom, Ireland and Canada, was arrested December 28 in Moscow and charged with espionage, according to his lawyer Vladimir Zherebenkov. His family rejects the accusation, asserting he was in Russia only for a vacation.

"The situation around the American Paul Whelan, who was arrested in Russia, is very serious," Deputy Foreign Minister Sergei Ryabkov told RIA Novosti, the agency reported.

"As for the possibility of any 'exchanges,' it is impossible and wrong to raise a question on this now, when even the charges have not been formally brought," he said.

Ryabkov's comments follow speculation in national security circles that Russian President Vladimir Putin or his government agencies might be looking to orchestrate some sort of swap for alleged Russian agent Maria Butina, who pleaded guilty to conspiracy charges in the United States just 15 days before Whelan's arrest.

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