Britain has warned Russia against playing “diplomatic chess games” following the arrest on spying charges by Moscow of Paul Whelan, a former U.S. Marine who also holds British and Irish citizenship.

"We don't agree with individuals being used in diplomatic chess games.... We are all extremely worried about him and his family," British Foreign Secretary Jeremy Hunt told the BBC on January 4.

Russia's Federal Security Service said on December 31 that it had detained Whelan while he was allegedly carrying out an act of espionage in Moscow.

At the time, he was identified only as an American.

His arrest comes with relations between Moscow and the West strained over a range of issues, including Russia’s role in wars in Syria and eastern Ukraine, its alleged meddling in elections in the United States and elsewhere, and the poisoning of a double agent in Britain.

"We are extremely worried about Paul Whelan. We have offered consular assistance," Hunt said. "The U.S. are leading on this because he is a British and American citizen."

Maria Zakharova, a spokeswoman at the Russian Foreign Ministry, confirmed that British officials have applied for consular access to Whelan.

Ireland's Department of Foreign Affairs on January 4 confirmed that Whelan also holds Irish citizenship and said it was also requesting consular access to him in Moscow.

Interfax said Russian authorities had brought formal charges against Whelan on January 3.

Whelan's lawyer, Vladimir Zherebenkov, did not comment on the charges but said that under the terms of his arrest, Whelan was expected to remain in pretrial custody in Moscow until at least February 28.

Zherebenkov said on January 3 that he had appealed the two-month pretrial custody order, asking for Whelan to be released on bail instead.

The U.S. State Department said U.S. Ambassador to Russia Jon Huntsman visited Whelan at a detention facility in Moscow on January 2 and spoke by phone with his family.

Whelan's family has denied he is a spy and said he was in Moscow to attend a wedding.

The family said Whelan was last heard from on December 28.

Whelan is a former U.S. Marine who lives in the state of Michigan and is director of global security at BorgWarner, a U.S.-based supplier of automotive parts and components.

The detention of Whelan comes weeks after Russian Maria Butina pleaded guilty in a U.S. court to acting as an agent for the Kremlin.

The Kremlin has denied that Butina is a Russian agent and has organized a social-media campaign to secure her release.

In the past, Russia has sometimes arrested foreigners with the aim of trading prisoners with other countries.

With reporting by AFP, Reuters, TASS, and AP