Washington -- The brother of a U.S. citizen arrested in Russia on espionage charges said Tuesday that he's innocent and was in Moscow to attend a wedding. Paul Whelan was arrested in Moscow on Friday.

The Russian Federal Security Service, in announcing the arrest three days later, said Whelan was caught "during an espionage operation," but gave no details.

"We are deeply concerned for his safety and well-being," his brother, David Whelan, said in a statement posted on Twitter. "His innocence is undoubted and we trust that his rights will be respected."

My brother was detained by the Russian government on Friday as an alleged spy. While the law library + info focus will remain, you may see an increase in off-message topics until we get him safely home. pic.twitter.com/2HIF1UmS1b — David Whelan (@davidpwhelan) January 1, 2019

The Russian spying charges carry a prison sentence of up to 20 years.

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He said the family last heard from Paul Whelan, a retired Marine, on Friday, "which was very much out of character for him, even when he was traveling."

The State Department said Monday it had received formal notification from the Russian Foreign Ministry of the arrest and was pushing for consular access. The department did not identify Whelan at the time or provide any information about the case, citing "privacy considerations."

The arrest comes as U.S.-Russian ties are severely strained, in part over Russian meddling in the 2016 presidential election.

A Russian gun-rights activist, Maria Butina, is in U.S. custody after admitting she acted as a secret agent for the Kremlin in trying to infiltrate conservative U.S. political groups as Donald Trump was seeking the presidency. She pleaded guilty in December to a conspiracy charge as part of a deal with federal prosecutors.

Russian President Vladimir Putin has claimed the case is fabricated and that Butina entered the guilty plea because of the threat of a long prison sentence.