(NEW YORK) — The family of Paul Whelan, the American former Marine held hostage by Russia, say they are worried for his safety after he dropped out of contact at the prison camp where is being held last week.

Whelan’s brother, David Whelan, on Tuesday said camp authorities had claimed they had abruptly moved Whelan to a prison hospital without explanation. In a statement, he said the family fear Whelan could have suffered a sudden medical emergency or that the prison authorities might be lying to conceal that Whelan was now in solitary confinement or otherwise held and not allowed to communicate.

Paul Whelan has spent nearly four years in detention since he was seized in 2018 by Russia’s domestic intelligence agency, the Federal Security Service, while visiting Moscow for a friend’s wedding. He was sentenced to 16 years in prison on espionage charges but the United States and his family say were fabricated in order to take him as a political bargaining chip.

Whelan has been held in at Correctional Colony 17 — a prison camp in the Mordovia region around 300 miles from Moscow — for more than two years. His family began sounding the alarm on Monday after Whelan missed scheduled calls with his parents and the U.S. embassy last week.

“We’re concerned that he may have either some emergency medical issue that is not being disclosed or that he is in fact still at IK-17 and has been placed in solitary as retaliation for something,” David Whelan said in an email statement.

David Whelan said the prison camp claims his brother was moved to the prison hospital on Nov. 17 but Whelan had not complained of any medical issue to U.S. embassy staff that visited him a day earlier or mentioned any move to his parents in a call on Nov. 23.

“Paul was not complaining of any health conditions that required hospitalization, so has there been an emergency? He appeared healthy and well to the Embassy staff,” David Whelan wrote.

“If Paul’s at the prison hospital, why is he being prohibited from making phone calls that every prisoner is allowed to make? Is he unable to make calls? Or is he really still at IK-17 but he’s been put in solitary and the prison is hiding that fact,” he wrote.

He said the prison had previously sent Whelan to the hospital against his will, sometimes as a punishment. Previously, however, Whelan noted that his brother had always told them when he was being sent to the hospital, David Whelan said.

Whelan’s Russian lawyer, Vladimir Zherebenkov, told ABC News that Whelan had a medical check up two weeks ago which suggested no issues.

Zherebenkov said there were two explanations why Whelan was not making his calls — either there are technical issues or he was being punished.

Asked by ABC News on Tuesday, the spokesman for the White House National Security Council, John Kirby, said it was the first he had heard Whelan had missed a call but he expressed deep concern for Whelan’s conditions.

Whelan is able to speak most weeks to his parents in the U.S. via video call from the prison, according to his family, and also has scheduled calls to the embassy.

“They have a call scheduled with him on Thursdays and he failed to make it last week. It’s incredibly unusual for Paul to miss trying to call home on a holiday like Thanksgiving,” David Whelan said. He said he hoped the U.S. embassy would find out Whelan’s condition.

“Is his phone card out of funds? Is he in solitary? Has he been moved to a hospital camp again without his request? Transport somewhere always comes to mind because prisoner transfers always seem to happen on a Friday. And, if it’s punitive, what is the prison retaliating for?”

The U.S. is seeking to negotiate Whelan’s release as well as the release of WNBA star Brittney Griner, who has been in Russian detention since February and who American officials also believe was taken by the Kremlin as a political bargaining chip.

Griner arrived earlier this month at a nearby prison camp in Mordovia after she was sentenced to nine years on drug smuggling charges, the U.S. says were trumped up.

Russia has signaled it wants to trade Whelan and Griner in a prisoner exchange for Russians imprisoned in the U.S., but efforts to reach a deal have stalled.

The Biden administration this summer said it was prepared to trade Viktor Bout, the notorious arms trafficker who is serving a 25-year prison sentence on weapons smuggling charges and is widely suspected to have ties to Russian intelligence. But Russia has so far rejected the offer, despite publicly suggesting for years Bout was a top candidate for any swap.

Russia’s deputy foreign minister Sergey Ryabkov earlier this month said he hoped the prospect of trading Bout was “getting stronger” but that the two sides were “yet to arrive at a common denominator.” He confirmed the U.S. and Russia were negotiating on the issue via a “special channel” and that Bout was among those being discussed.

A potential sticking point is Moscow may be their unwillingness to trade two Americans for one Russian citizen, even one of Bout’s importance.

In new comments on Tuesday, Ryabkov said “there is always a chance” that a deal might be reached soon, but gave little suggestion one was close.

“Regretfully, we have seen a number of situations where we thought a decision would be made soon. That did not happen,” Ryabkov told reporters.

Ryabkov also criticized the U.S. for speaking publicly about the negotiations, accusing it of “going over the top by using megaphone diplomacy.”

The acting U.S. ambassador to Moscow, Elizabeth Rood, in an interview with Russian state media this week confirmed that the U.S. was continuing to discuss Griner and Whelan through special channels.

“We have already said, the United States has submitted a serious proposal for consideration. We finalized this proposal and offered alternatives. Unfortunately, the Russian Federation has not yet received a serious response to these proposals,” she told RIA Novosti.

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