A Moscow court found Whelan guilty of receiving classified information as he stood in the dock with a sign that read "Sham trial!" and pleaded for US President Donald Trump to intervene.

The court process is "slimy, greasy Russian politics, nothing more, nothing less," he told reporters.

The case has strained ties between Moscow and Washington and fuelled speculation of a prisoner swap.

Whelan, 50, has been held in a Moscow jail since his arrest in December 2018 and his trial was held behind closed doors because of the accusations of seeking to obtain state secrets.

He had complained of being mistreated in prison and last month underwent an urgent hernia operation at a Moscow hospital.

Whelan's conviction is another impediment in relations between the two world powers, which are at odds over Ukraine, Syria, Libya, arms control and a host of other issues.

US Secretary of State Mike Pompeo slammed the verdict and demanded that Russia release Whelan immediately.

He said the United States was outraged "after a secret trial, with secret evidence, and without appropriate allowances for defence witnesses".

The US ambassador to Moscow John Sullivan condemned the trial as unfair and lacking transparency, saying he was "disappointed, outraged" by the verdict.

He said Whelan was "sentenced to 16 years in prison with no evidence that's been produced."

"I can't say I'm surprised," he added. "This is the direction this case has been heading from the beginning."

- 'They abducted Mr Bean' -

Kremlin spokesman Dmitry Peskov dismissed allegations Whelan's detention was political saying: "The charges brought against him were proven in court and accepted by the court."

Throughout the hearings, Whelan insisted he was innocent, saying he was framed and comparing himself to the hapless comedy character Mr Bean.

"Russia thought they caught James Bond on a spy mission, in reality they abducted Mr Bean on holiday," he said in October.

Whelan says he was detained on a visit to Moscow to attend a wedding when he took a USB drive from an acquaintance thinking it contained holiday photographs.

Whelan, who also has British, Canadian and Irish passports, was sentenced to 16 years in a strict regime penal colony.

The prosecution had asked for 18 years, just short of the maximum 20-year term for the charge.

The prosecutor said last month Whelan tried to receive information that was a state secret and could harm Russia's security, Whelan's lawyer Vladimir Zherebenkov told reporters.

The prosecutor claimed Whelan has the rank of an officer -- "at least a colonel" -- at the US defence intelligence agency, the lawyer said.

- 'Russian judges are political' -

Whelan's brother David said his family would continue to fight for his release.

"We had hoped that the court might show some independence but, in the end, Russian judges are political, not legal, entities," he said.

Whelan was dishonourably discharged as a marine before working as head of global security at a US auto parts company.

There had been suggestions that Whelan could be returned to the US in a prisoner swap.

Whelan's lawyer said Monday that Russian intelligence services were considering an exchange for two Russians detained in the United States, including pilot Konstantin Yaroshenko, imprisoned on drug smuggling charges.

David Whelan has said that the family hoped a conviction would allow Russia and the United States to begin discussing Paul's release immediately.

US diplomats and the former marine's family have condemned Whelan's treatment in detention, saying Russia ignored his medical condition and barred him from communicating with relatives.

The US embassy criticised the Russian authorities for waiting until his condition was "life-threatening to provide him medical attention", while the Russian foreign ministry described his operation last month as "simple".