Pro-Russian rebels released nine abducted Red Cross workers on Saturday in Donetsk.
The nine - 8 Ukrainians and a Swiss - had been detained on Friday evening.
"Now they will take up their humanitarian work and prepare a medical evaluation," said David Pierre Marquet, a spokesman for the International Committee of the Red Cross.
A spokesman for the self-styled "Donetsk People's Republic," whose proponents plan an internationally-rejected separatist referendum on Sunday, said the Red Cross workers had been accused of espionage by rebels.
The internet site Novosti Donbassa said the hostage-takers had also seized large stocks of medicine that the Red Cross had recently sent to the Donetsk region amid fighting between pro-Russian militants and Ukrainian security forces.
A local Red Cross official in Donetsk said one of the hostages freed had been severely beaten.
Fighting in eastern port city
The overnight incident followed chaotic fighting on Friday in the center of Mariupol, eastern Ukraine's port city, between forces of Ukraine's Kiev-based interim government and pro-Russian rebels seeking succession for eastern Ukraine.
Regional authorities said seven people had been killed in Mariupol and 39 wounded.
Ukrainian interior minister Arsen Avakov said about 20 people whom he described as "terrorists" and a soldier had been killed.
The clashes took place around the city's police headquarters.
Ukrainian forces appeared to have withdrawn on Saturday from the industrial city, which has a population of half a million.
Secessionists to go ahead with referendum
Pro-Russian secessionists plan voting on Sunday at 53 locations across the rebel zones of Donetsk and Luhansk, despite Western demands that Ukraine await a country-wide presidential election planned for May 25.
The news agency Reuters said the list of voters was two years old; there was no minimum turnout stipulated; and outside observers had not been invited.
There was also uncertainty about what the question on the ballot paper meant:
"Do you support the act of self-rule of the People's Republic of Donetsk?"
Kiev warns of social, economic dislocation Ukrainian acting President Oleksander Turchinov on Saturday urged eastern regions to accept "round table" talks on greater autonomy but to avoid the secessionist referendum.
He said a "yes" vote on Sunday would thrust the eastern regions "into the abyss."
Independence proponents, he said, "don't understand that this would be a complete destruction of the economy, social programs and general life for the majority of the population."
Putin's Crimea visit condemned
On Friday, Russian President Vladimir Putin visited Crimea, which Russia annexed in March despite international condemnation, claiming that his administration had "re-established the historic truth."
His visit was condemned as "inappropriate" by the Western military alliance NATO.
The United States said Putin's leading of a military parade in Crimea - commemorating the Soviet World War II victory over Nazi Germany - was provocative.
"An important day in our shared history, dedicated to honouring the enormous sacrifices and giving remembrance to the millions of dead in the Second World War, should not have been instrumentalised to give visibility to the illegal annexation of Crimea," said a spokeswoman for EU foreign policy chief Catherine Ashton.
The EU is likely to extend its selected sanctions against Russia on Monday.
European commission member Janusz Lewandoski said Brussels would target about 15 people and several Crimean branches of Ukrainian companies taken over by Russians.
Crimea became part of Ukraine in 1954 and remained in independent Ukraine after the 1991 collapse of the Soviet Union.
Source: Deutsche Welle