A Russian military court on Tuesday sentenced a Ukrainian filmmaker to 20 years in a strict-regime prison colony on terror charges in a trial condemned by Kiev, Western rights groups and top film directors.

Oleg Sentsov, 39, was convicted for allegedly carrying out arson attacks on pro-Kremlin party offices in Crimea after it was seized by Russia last March, and plotting further attacks, including blowing up a Lenin statue in the peninsula's main city of Simferopol.

His fellow Ukrainian co-defendant Alexander Kolchenko was sentenced to 10 years for allegedly taking part in the attacks.

Acclaimed filmmakers from across the globe, including Spain's Pedro Almodovar and Britain's Mike Leigh, have written to Russian President Vladimir Putin expressing concern over Sentsov's prosecution.

Russian film director Andrei Zvyagintsev, whose latest film "Leviathan" won a Golden Globe, wrote in a letter published in Novaya Gazeta newspaper on Monday that it was "monstrous to jail a young man, a promising filmmaker".

Zvyagintsev called for Russia to "either release him or only try him for what you can prove irrefutably".

Sentsov had his debut feature "Gamer" shown at the Berlin Film Festival in 2012.

As the sentence was read, the up-and-coming filmmaker defiantly flicked a victory sign and he and Kolchenko sang the Ukrainian national anthem inside their glass enclosure.

- 'The height of injustice' -

Ukrainian President Petro Poroshenko, who had called for the men's immediate release, wrote on Twitter in Ukrainian: "Hold on, Oleg. The time will come when those who organised the trial against you will find themselves in the dock."

Sentsov's lawyer Dmitry Dinze said that his client would appeal his sentence.

"It's the height of injustice and lawlessness," TASS state news agency quoted Dinze as saying.

Sentsov and Kolchenko have been held behind bars in Russia since May last year.

The case was heard in a military court in the southern city of Rostov-on-Don, one of only two courts in Russia authorised to hear terrorism cases.

Prosecutors last week asked for Sentsov to be jailed for 23 years and for Kolchenko, a pro-Kiev activist who opposed Russia's annexation of Crimea, to be sentenced to 12 years.

The men were tried as Russians, despite never having applied for citizenship. Both pleaded not guilty.

In his final trial statement, Sentsov condemned Moscow's rule.

"Your propaganda is very good, but there are also people like you who understand very well that there are no 'fascists' in Ukraine, that Crimea was taken illegally and that your troops are in Donbass," he said of the conflict zone in eastern Ukraine.

Dinze told AFP when the trial started last month that he expected a guilty verdict but hoped the men could be returned to Ukraine in a prisoner exchange.

The men are among 11 Ukrainians held in Russian prisons whom Kiev considers to be political prisoners, according to Ukrainian Foreign Minister Pavlo Klimkin.

These include military pilot Nadiya Savchenko who is currently on trial over the deaths of two Russian journalists in eastern Ukraine.

Russian President Vladimir Putin has denied that Sentsov and Savchenko are prisoners of war.

- 'Fatally flawed' -

Amnesty International condemned the case, saying it was "fatally flawed" in a statement released after the verdict.

It called the long sentences a "blatant injustice after a patently unfair trial marred by credible allegations of torture."

Defence lawyers say witnesses have been tortured to produce testimony implicating Sentsov and Kolchenko in activities involving Ukrainian far-right organisation Right Sector, which is banned in Russia.

Two alleged accomplices of Sentsov have already been sentenced to seven-year terms after confessing guilt. However they both refused to testify after being called as prosecution witnesses in Sentsov's trial, with one saying he had earlier given testimony under duress.