STORY: Presidium Network, which does relief work in Ukraine, including help with evacuations, said the release of the five prisoners had happened faster than the non-profit expected.
"We've been kept in the dark on a lot of these political negotiations for obvious reasons. So it became a surprise because we were still preparing for the next trial in October," said Dominik Byrne, the co-founder of Presidium Network.
"The five that were released, they were supposed to be back in court in October and we were expecting that to be the next phase," he added.
The five are Aiden Aslin, Shaun Pinner, John Harding, Dylan Healy and Andrew Hill.
Aslin and Pinner were captured by Russian-backed forces in Ukraine's coastal city of Mariupol in April and were sentenced to death by a court in the self-proclaimed Donetsk People's Republic, one of Russia's proxies in eastern Ukraine.
They, along with Moroccan Brahim Saadoun, were found guilty of "mercenary activities and committing actions aimed at seizing power and overthrowing the constitutional order of the DPR" -- a sentence condemned by Ukraine and Britain.
Healy, a Briton providing humanitarian assistance in Ukraine, was detained in April alongside Paul Urey, who later died in detention. Harding and Hill had been fighting alongside Ukrainian forces and all three had denied being mercenaries in the Russian proxy court.
Byrne told Reuters the five men had been transported back to London through Saudi Arabia. All five have been reunited with family and will undergo a "reintegration process", he said.