MOSCOW/KIEV (REUTERS, AFP) - Russia and Ukraine carried out a long-awaited prisoner swap on Saturday (Sept 7) in a step that could thaw a deep freeze in relations since Moscow's annexation of the Crimea region in 2014.

While the exchange of 35 prisoners on each side could help rebuild confidence between Moscow and Kiev and allow them to start negotiating seriously on other issues, any road to a full rapprochement is likely to be long and complex.

After lengthy negotiations, expectations have been running high for the prisoner swap, which had been described as imminent by the leaders of both countries in recent days.

On Saturday a Russian aircraft carrying freed Russian prisoners from Kiev landed in Moscow while a Ukrainian plane with released Ukrainian prisoners aboard touched down in Kiev.

Ukrainian President Volodymyr Zelenskiy hugged and shook hands with the freed Ukrainians while Russia's Rossiya 24 TV showed Russian prisoners disembarking the plane in Moscow.

Mr Zelenskiy told reporters at the Kiev airport that the swap was part of his deal with Russian President Vladimir Putin.

He said all steps had to be taken "to finish this horrible war," referring to the five-year-old conflict with pro-Russian separatists in eastern Ukraine.

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The swap was carried out on a "35-to-35" basis, Interfax news agency quoted Russian human rights commissioner Tatyana Moskalkova as saying. "Ukraine has handed out 35 people to Russia, we handed out the same number to Ukraine," she said.

"We view the agreed mutual release of persons held in Russia and Ukraine as a positive signal that should be followed by other important steps to break the impasse in the current situation in Russia-Ukraine relations," the Russian Foreign Ministry said in a statement.

Though Ukraine's demands for the return of Crimea have fallen on deaf ears in Russia, Saturday's exchange is seen in some quarters as a win for Mr Zelenskiy, who swept to power this year promising to repatriate compatriots held in Russia.

The US Embassy in Kiev congratulated Mr Zelenskiy on bringing home the Ukrainian citizens and called on Russia to release "the many other Ukrainians who remain unjustly in Russian custody," it said in a social media post.

US President Donald Trump congratulated Russia and Ukraine on the prisoner swap, saying in a tweet that the move was “very good news, perhaps a first giant step to peace.”

Russian-backed separatists continue to control a swathe of Ukraine's east in a conflict that has killed more than 13,000 people. Mr Zelenskiy has pledged to end the regular, low-level clashes that persist despite a ceasefire signed in 2015.

The swap could set the stage for serious talks, even though major differences remain between the two countries. French President Emmanuel Macron has been pushing for a summit to discuss the issue with Russia, Ukraine, Germany and France.

Mr Putin said on Thursday the exchange would be "a good step forward towards the normalisation (of relations)" with Russia's fellow former Soviet republic, adding he expected large numbers of prisoners to be involved.

Among the Ukrainians freed on Saturday were 24 sailors detained by Russia during a clash in waters off Crimea last year. Ukrainian filmmaker Oleg Sentsov, jailed in Russia, was also in the group.

Among prisoners handed over to Moscow was Volodymyr Tsemakh, suspected of involvement in the downing of a Malaysia Airlines MH17 flight over rebel-held eastern Ukraine in 2014 that killed all 298 aboard.

The airliner was on a flight from Amsterdam to Kuala Lumpur, and the Dutch government issued a statement saying it "seriously regrets that under pressure from the Russian Federation Mr Tsemakh was included in this prisoner swap".

Dutch prosecutors did get a chance to question Mr Tsemakh before his release, it added.

Ukraine's security service has identified Mr Tsemakh as a former commander of Russian-backed separatist forces in eastern Ukraine.

A senior Ukrainian official said Russia refused to proceed with an exchange of prisoners unless Ukraine agreed to hand over Mr Tsemakh, who was reportedly in charge of air defence in the area where the MH17 came down.

“Tsemakh’s absence from the exchange list automatically meant the cessation of talks with Russia,” Mr Ivan Bakanov, the head of Ukraine’s SBU security service, told the Interfax-Ukraine news agency.

Mr Bakanov described Moscow’s insistence on his release as “proof of Russia’s involvement in the MH17 crash”.

He said the Ukrainian government had no “moral right” to put off the swap any longer. “As a state, Ukraine is showing the world that it is saving its citizens like the world’s strongest democracies,” he said.

German Chancellor Angela Merkel on Saturday hailed the prisoner swap as a "sign of hope" and called for the implementation of a 2015 ceasefire deal that Paris and Berlin helped broker.

"This exchange of prisoners between Russia and Ukraine is a sign of hope... it's worth continuing the hard work to implement the Minsk accord," Dr Merkel said in a statement put out by her spokesman Steffen Seibert on Twitter. "I am happy for the Ukrainian sailors and Oleg Sentsov who can finally return home," she said.

The 43-year-old Mr Sentsov was the most famous Ukrainian prisoner held in Russia and the subject of a star-studded international campaign calling for his release. He was arrested in 2014 and had been serving a 20-year sentence in an Arctic penal colony for planning "terrorist attacks" in Crimea.