Pro-Russia forces on Sunday freed one of the eight European military observers being held prisoner in eastern Ukraine.
The Swedish observer was escorted to an Organization for Security and Cooperation in Europe (OSCE) vehicle and driven away.
A Reuters reporter outside the city administration building in Slovyansk said the man came out escorted by three unarmed men, got into a white OSCE jeep and drove off.
Earlier on Sunday, a group of European monitors detained by pro-Russia insurgents in eastern Ukraine appeared in public Sunday to give assurances they are not being mistreated, even as negotiations began to secure their release.
The foreign observers are from Germany, Poland, Sweden, Denmark and the Czech Republic.
They were shown to the media on Sunday, led into Sloviansk town hall by masked gunmen.
German monitor Col Axel Schneider, who spoke for the group, stressed they were not NATO officers – contrary to claims made by the separatists – nor armed fighters, but diplomats in uniforms.
"We are not prisoners of war. We are the guests of (self-declared Sloviansk) Mayor (Vyacheslav) Ponomaryov, and being treated as such."
In a separate development on Sunday, pro-Russian gunmen said they had captured three Ukrainian security services members overnight in eastern Ukraine. Kiev later confirmed a number of its officers had been seized.
In eastern Ukraine, gunmen continue to occupy official buildings in a dozen cities, defying the government in Kiev.
Russia, an OSCE member, earlier pledged that it would "take all possible steps" to secure the release of the observers.
The Group of Seven rich countries have agreed that they will impose new sanctions on Russia over the crisis in Ukraine, a statement said Saturday, with a senior US official saying they could come as early as Monday.
In a joint statement
released late Friday, the G-7 nations (- the U.S., Canada, Britain, France, Gernamy, Italy and Japan -) said they would take measures to intensify "targeted sanctions."
"Given the urgency of securing the opportunity for a successful and peaceful democratic vote next month in Ukraine\’s presidential elections, we have committed to act urgently to intensify targeted sanctions and measures to increase the costs of Russia\’s actions," said the statement.
The US and EU already has assets freezes and travel bans in place target a number of Russian individuals and firms accused of playing a part in the annexation of Crimea.
Rebel militia continue to occupy official buildings in a dozen eastern cities, defying the government in Kiev.
Russian planes have violated Ukrainian airspace several times over the past 24 hours, a Pentagon spokesman said Friday.
Pentagon spokesperson Colonel Steven Warren confirmed the breach on Friday, but did not provide more details, includign where the incidents occured or what kind of Russian planes were involved.
He urged Russia to take "immediate steps to de-escalate the situation," AFP news agency reported.
With tensions soaring, US Defence Secretary Chuck Hagel had tried to arrange a phone conversation with his Russian counterpart, Sergei Shoigu, to discuss the crisis, but Moscow has yet to respond to the request, Warren said.
Slovyansk, a city of around 130,000, has been for two weeks under the control of separatists who, like similar groups elsewhere in eastern Ukraine, oppose the central government in Kiev after the overthrow of a Kremlin-backed president.