Secretary of State Mike Pompeo on Monday said the U.S. is "outraged" over Russia's conviction of American Paul Whelan on spying charges, slamming the trial that led to a conviction and a 16-year prison sentence as illegitimate.

Both Whelan and his family have said that he is innocent, and was in Russia at the time of his arrest to attend the wedding of a friend from his time in the Marines. The United States has for months raised concerns about the lack of due process for Whelan and his deteriorating health in Russian custody, and Pompeo personally demanded Whelan's release in a May 30 tweet. He did so again Monday.

"Outraged by the decision today to convict Paul Whelan on the basis of a secret trial, with secret evidence, and without appropriate allowances for defense witnesses," Pompeo tweeted Monday. "Paul’s treatment by Russian authorities continues to be appalling, and we demand his immediate release."

AMERICAN PAUL WHELAN RECEIVES 16-YEAR RUSSIAN JAIL SENTENCE: REPORT

Pompeo elaborated further in a separate statement Monday.

"We have serious concerns that Mr. Whelan was deprived of the fair trial guarantees that Russia is required to provide him in accordance with its international human rights obligations," he said. "Russia failed to provide Mr. Whelan with a fair hearing before an independent and impartial tribunal; and during his detention has put his life at risk by ignoring his long-standing medical condition; and unconscionably kept him isolated from family and friends."

America's top diplomat also said the U.S. demands "Paul Whelan's immediate release," in the longer statement.

Paul Whelan, a former U.S. Marine accused of espionage and arrested in Russia in December 2018, stands inside a defendants' cage as he waits to hear his verdict in Moscow on June 15, 2020. (Photo by KIRILL KUDRYAVTSEV/AFP via Getty Images)

Paul Whelan, a former U.S. Marine accused of espionage and arrested in Russia in December 2018, stands inside a defendants' cage as he waits to hear his verdict in Moscow on June 15, 2020. (Photo by KIRILL KUDRYAVTSEV/AFP via Getty Images)

GEN. JACK KEANE ON RUSSIAN BOMBERS INTERCEPTED BY US FIGHTER JETS: 'THEY ROUTINELY TEST OUR DEFENSES'

Whelan, who was arrested in December 2018, has said that he was set up — that a person he considered a friend handed him a USB drive allegedly containing vacation photos. But minutes later Russian authorities stormed his hotel room. Russia says that the drive contained a list of names of those working at a classified security agency.

Whelan's family, through his brother David, also released a statement that slammed Russia's legal system as "guilty of injustice," the investigator on the case as "incompetent," the prosecutor as bringing "the legal profession into disrepute," and Russian judges as "political."

The statement also called on the U.S. and President Trump in particular to "swiftly" move to bring Whelan back to the U.S.

"We look to the U.S. government to immediately take steps to bring Paul home," it said. "We will continue to work with members of Congress and the State Department to advocate for Paul's freedom and protect his human rights until he is free.  We will rely on the help of David Urban, who is able to have conversations in the White House that our family cannot.  And we will look to President Trump, who alone can act to bring Paul home.  We hope he will do so swiftly."

U.S. Ambassador to Russia John Sullivan also Monday slammed Russia over its treatment of Whelan.

"Ambassador Sullivan: The United States demands that U.S. citizen #PaulWhelan be released immediately. His conviction is a mockery of justice. The world is watching," Sullivan's spokeswoman, Rebecca Ross, tweeted.

'FOREIGN ADVERSARIES' TARGETING US OVER GEORGE FLOYD UNREST, INTELLIGENCE BULLETIN WARNS

The official Russian-language Twitter account of the U.S. Embassy in Russia posted an identical message.

Whelan, according to the BBC, has said that his detention in Russia constitutes a "hostage situation" and that "Russia says it caught James Bond on a spy mission. In reality, they abducted Mr. Bean on holiday."

Whelan previously told AFP it's possible he's being held as a pawn in a potential prisoner swap between the U.S. and Russia. The Russian Foreign Ministry denied such claims.

Sullivan late last month, in a video posted to the U.S. Embassy in Russia's website, tore into the country's treatment of Whelan, who is also a citizen of Canada, Ireland and the United Kingdom.

"We've watched while Paul has denied due process, while he's been denied access to the outside world, to his family and friends, while he's been denied outside medical care as his health continues to deteriorate, and at every opportunity we've spoken out," Sullivan said. "So little has been done by Russian authorities to ensure Paul's basic human rights."

Sullivan added: "The Russian government has allowed no evidence — none. All court hearings are being held behind closed doors, in secrecy away from the media, from the public and from me. Away from anyone who might challenge their definition of 'red-handed.' There's been talk of evidence, allusion to evidence and even allusions to discussion of evidence.

CLICK HERE TO GET THE FOX NEWS APP

"But where is the evidence itself? Hearings are taking place behind closed doors in secrecy. Details of alleged witnesses and alleged evidence appear in the press, but we do not know what is happening behind closed doors. Why so much secrecy?" Sullivan said.

David Whelan on Monday also tweeted about the sentencing, thanking American diplomats for their support.

"We hope the @RealDonaldTrump Administration will now take action to bring Paul home," he said in one tweet.

"Once again, our family owes an unpayable debt to the @USEmbRu @USEmbRuPress staff and for Ambassador John Sullivan's focus on getting justice - and freedom - for #PaulWhelan," David Whelan said in another.

Fox News' Morgan Phillips and Paulina Dedaj, and The Associated Press contributed to this report.