Concern grows about Paul Whelan, an American imprisoned in Russia.

The Biden administration is “deeply concerned” about Paul Whelan, an American imprisoned in Russia who has been transferred to a prison hospital, a White House spokesman said on Wednesday.

Mr. Whelan’s brother, David, said in emails to supporters this week that his brother was moved on Nov. 17 to a hospital in the prison where he is being held.

His family, which has not heard from him in a week, grew particularly alarmed when Mr. Whelan missed a scheduled call home on Thanksgiving Day and further still when he failed to call home on Wednesday, his father’s 85th birthday.

“Paul was not complaining of any health conditions that required hospitalization, so has there been an emergency?” David Whelan wrote. He added that his brother “appeared healthy and well” to U.S. embassy staff who visited him earlier this month.

John Kirby, a National Security Council spokesman, told reporters in a telephone briefing on Wednesday that the U.S. government has been trying unsuccessfully to get information on Mr. Whelan’s condition and his whereabouts.

“As we speak this morning, regrettably, we do not have an update specifically about where he is or what condition he’s in,” Mr. Kirby said. He added: “We are deeply concerned about the lack of information and the lack of contact from Paul, and we’re working on this really as hard as we can through diplomatic channels.”

Speaking on MSNBC during a visit to Bucharest, Romania, Secretary of State Antony J. Blinken said that U.S. officials had visited Mr. Whelan on Nov. 16 and spoke to him by phone at “roughly” the same time but have not had contact with him since. “We are working every day to make sure that we have contact with him, that we understand what the exact situation is,” Mr. Blinken said.

David Whelan said in an email on Wednesday: “It could be nothing but, in this case, you always have to consider worst case scenarios.”

Paul Whelan, a former U.S. Marine who later worked as a corporate security executive, was arrested at a Moscow hotel in December 2018 and convicted in June 2020 on espionage charges that the U.S. government says were manufactured.

U.S. officials have linked Mr. Whelan’s fate to that of the imprisoned professional basketball player Brittney Griner and have been trying for months to negotiate their joint release. The United States has offered to release from federal prison a notorious Russian arms dealer, Viktor Bout, in exchange for the two Americans. Russia has yet to accept the offer.

In a Nov. 9 statement, the White House said that it “has continued to follow up on that offer and propose alternative potential ways forward with the Russians through all available channels” but did not offer further details.

Mr. Whelan is being held at a high-security prison called IK-17, about an eight hour drive from Moscow.