Paul Whelan, a former United States marine arrested in Russia on espionage charges, was visiting Moscow to attend a wedding when he suddenly disappeared, according to his brother.

Key points: Paul Whelan failed to show up for the wedding he was due to attend in Moscow

Paul Whelan failed to show up for the wedding he was due to attend in Moscow The former US marine is the head of global security for an auto parts supplier

The former US marine is the head of global security for an auto parts supplier His arrest follows that of a Russian gun lobbyist in the US who admitted to being a Kremlin secret agent

Mr Whelan, 48, who is head of global security for a Michigan-based auto parts supplier, was arrested on Friday.

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

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

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

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David Whelan said in an interview that his brother had been to Russia several times previously, so when a fellow former marine was planning a wedding in Moscow with a Russian woman he was asked to come along to help out.

On the morning of his arrest, he had taken a group of wedding guests on a tour of the Kremlin museums.

The last time anyone heard from him was at about 5:00pm and he failed to show up that evening for the wedding, his brother said.

"It was extraordinarily out of character," he said.

The family feared he had been mugged or was in a car accident, David Whelan said, and it was when searching the internet on Monday that he learned of the arrest.

"I was looking for any stories about dead Americans in Moscow, so in a way it was better than finding out that he had died," he said.

The US State Department said it had received formal notification of the arrest from the Russian Foreign Ministry and was pushing for consular access.

Paul Whelan completed multiple tours in Iraq with the Marine corps, his brother said.

He now lives in Novi, Michigan, and has worked for BorgWarner since early 2017.

"He is responsible for overseeing security at our facilities in Auburn Hills, Michigan and at other company locations around the world," company spokeswoman Kathy Graham said in a statement. She said BorgWarner does not have any facilities in Russia.

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

A Russian gun-rights activist, Maria Butina, is in US custody after admitting she acted as a secret agent for the Kremlin, trying to infiltrate conservative US 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.

AP