A top Russian official said Wednesday that a U.S. citizen accused of espionage was caught "red-handed," Reuters reported.

“He was detained at the moment he was carrying out specific illegal actions in his hotel," Foreign Minister Sergei Lavrov said of Paul Whelan, who has been detained since last month. Lavrov did not elaborate, according to the news service.

Lavrov said U.S. diplomats would be allowed to visit Whelan for a second time, and that an Irish diplomat was set to meet soon with the former Marine, as well.

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Reuters reported that Lavrov said the investigation into Whelan was not motivated by a desire to try and swap him for Russian citizens held in the U.S.

Whelan, a former Marine who works as a security executive for automotive parts maker BorgWarner in Michigan, was detained late last month in Russia on suspicion of espionage.

Whelan's twin brother said his sibling was in Moscow for a wedding between a fellow ex-Marine and a Russian woman, and insisted that Whelan is innocent.

U.S. Ambassador to Russia Jon Huntsman visited with Whelan earlier this month in the Lefortovo Detention Facility.

Whelan's arrest comes amid heightened tensions between the U.S. and Russia.

Maria Butina, a Russian national, pleaded guilty in December to acting as an unregistered agent of the Russian government in the U.S. She faces a maximum of five years in prison.

The Senate voted Tuesday to advance legislation blocking the Trump administration's plan to lift sanctions against three Russian companies, bucking the president.