Members of Congress on Thursday introduced a bipartisan measure calling for the release of former U.S. Marine Paul Whelan, who has been detained in Russia since December on espionage charges amid deteriorating health and tough conditions.

A Russian court has repeatedly extended Whelan's detention in a cramped Moscow prison while he awaits trial on charges that carry a sentence of up to 20 years in prison. The U.S. Embassy said requests to send a doctor to examine him have been rejected.

"Russian authorities haven't come up with any evidence, any charges against him, which is simply unconscionable to be able to pick an American citizen off the street and put him in a prison — and not a very good prison at that," said Sen. Gary Peters, D-Mich. "After nine months, enough time has passed. It's time for action, it's time to release Paul."

Whelan was arrested outside a Moscow hotel on Dec. 28. He was in Russia to attend a friend's wedding.

Mr. President, we cannot keep America great unless we aggressively protect and defend American citizens wherever they are in the world. — Paul Whelan

Whelan, who also holds British, Irish and Canadian citizenship, denies the charges of spying for the U.S., which his lawyers say stem from a sting operation. Whelan's lawyer has said his client was handed a flash drive that had classified information he wasn’t aware of.

He has complained of poor prison conditions and has said his life is in danger, as a judge last month ordered him to remain in custody until at least October.

Whelan appealed to President Trump in June during an appearance at a Moscow courtroom, telling reporters: “Mr. President, we cannot keep America great unless we aggressively protect and defend American citizens wherever they are in the world.”

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Peters joined other members of the Michigan congressional delegation at a Capitol Hill news conference to announce a resolution calling on Moscow to present credible evidence against Whelan — or immediately release him from jail.

“Not to be able to produce any sort of charges or any sort of evidence is simply not the way a government should operate,” Peters said. “We believe the Russians should release Paul unless they come up with some sort of concrete evidence.”

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The bi-partisan resolution announced Thursday also urges the Russian government to provide unrestricted consular access to Whelan and to ensure that he is given due process.

The Associated Press contributed to this report.