One of the eight European observers being detained by pro-Russian separatists in the eastern Ukraine city of Slovyansk on Sunday evening was escorted to an Organization for Security and Cooperation in Europe, or OSCE, vehicle and driven away.
Stella Khorosheva, a spokeswoman for the separatist mayor of Slovyansk, said the observer who left is a Swedish national. "He has a mild form of diabetes and so we decided to let him go," she told reporters.
Asked if he was the only one of the observers who was to be released on Sunday, she replied: "Yes."
Pro-Russian rebels seized eight European OSCE monitors three days ago and have been holding them at their heavily fortified redoubt in the town of Slovyansk.
The captives, from Germany, the Czech Republic, Denmark, Poland and Sweden, were paraded before reporters on Sunday and said they were in good health.
"We have no indication when we will be sent home to our countries," the group's leader, German Colonel Axel Schneider, told reporters as armed men in camouflage fatigues and balaclavas looked on. "We wish from the bottom of our hearts to go back to our nations as soon and as quickly as possible."
Germany denounced the appearance and said Moscow must press their captors to free the prisoners.
"The public parading of the OSCE observers and Ukrainian security forces as prisoners is revolting and blatantly hurts the dignity of the victims," Foreign Minister Frank-Walter Steinmeier said in a statement.
"It is an infringement of every rule of behaviour and standards that are made for tense situations like this. Russia has a duty to influence the separatists so that the detained members of the OSCE mission are freed as soon as possible."
The OSCE, a European security body, includes Russia. Its main Ukraine mission was approved by Moscow, although the Europeans held in Slovyansk were on a separate OSCE-authorised mission that did not require Russia's consent.
Vyacheslav Ponomaryov, the rebel leader who has declared himself mayor of Slovyansk, has described them as prisoners of war and said the separatists were prepared to exchange them for fellow rebels in Ukrainian custody.