Paul Whelan, 50, was detained in Moscow in December 2018 for allegedly receiving state secrets but insists he was framed when he took a USB drive from an acquaintance thinking it contained holiday photos.

His trial, which has caused tensions between Moscow and Washington and sparked speculation of a prisoner swap, concluded with closing arguments on Monday.

Whelan's lawyer told reporters after the session that prosecutors had requested 18 years for his client in a penal colony, just short of the maximum 20-year sentence he was facing.

"To be honest, we are in shock," Vladimir Zherebenkov said outside the court.

Whelan reacted calmly to the prosecutor's "very harsh" demand, the lawyer said. "He is behaving with dignity," he added.

Zherebenkov said that the prosecutor believed Whelan to be an officer -- "at least a colonel" -- at the US Defense Intelligence Agency (DIA).

Whelan told the court "he did not do any spying and was not collecting any secret information," his lawyer said.

He added that the sentence would be delivered on June 15.

The trial, which began in March, has continued behind closed doors in a Moscow courtroom despite the coronavirus pandemic and diplomatic protests.

Whelan, who was head of global security at a US auto-parts supplier, said he was visiting Russia to attend a wedding at the time of his arrest.

He claimed that evidence he provided was ignored and the court was biased in favour of the prosecution and security services.

Whelan used earlier court hearings to appeal to journalists and US President Donald Trump, claimed he was being mistreated, not given full translations of documents and rarely granted access to his lawyer.

Whelan also holds Irish, Canadian and British citizenship.

- 'Intolerable' treatment -

His brother David said ahead of Monday's hearing that the family expected a guilty verdict.

"In a just system, the court would acquit Paul based on the lack of evidence," he said in a statement. "But we expect a wrongful conviction and can only hope that the sentence is at the lighter end of the range."

The US has condemned Whelan's detention saying there was insufficient evidence to hold him.

US Ambassador to Russia John Sullivan criticised Whelan's treatment in detention last month, saying it was "intolerable" that he was being barred access to medical care and has not been allowed to speak to family.

Russian authorities have blocked journalists and embassy employees from attending recent hearings because of the coronavirus epidemic.

Zherebenkov said earlier that three defence witnesses failed to show up to a hearing over coronavirus fears, saying they did not want to play "Russian roulette" and risk their lives by attending.

Whelan's case has raised speculation that the United States and Russia could be positioning themselves for a prisoner swap, possibly involving Russian pilot Konstantin Yaroshenko, imprisoned in the United States on drug smuggling charges.

Hopes for Whelan's release in exchange for Maria Butina -- a Russian woman arrested in the US in 2018 on espionage charges -- were quashed after Butina was flown to Moscow in October last year.

The case has exacerbated underlying tensions between Russia and the United States, which imposed sanctions on Moscow over its support for separatists in eastern Ukraine and the 2014 annexation of Crimea.