The rise in the number of prisoners being held by the militants—whose power base has emerged in the southeast city of Slovyansk—has given the anti-Kiev uprising the appearance of an armed conflict zone, despite the Kremlin's description of the activists in the east as everyday "citizens driven to desperation."
The militants demand a referendum on the southeast Donetsk region's future and denounce as illegitimate the pro-Europe authorities in Kiev that toppled President Viktor Yanukovych, a Donetsk native, in late February.
The Kremlin also refuses to recognize the new Kiev authorities and describes them as perpetrators of an armed coup.
The U.S. and European Union are preparing to impose a new round of sanctions against Russia this week as punishment for its annexation of Crimea and what Western officials have described as Kremlin support of the unrest in eastern Ukraine.
But the sanctions largely have failed to change Russia's stance on Ukraine, a former Soviet republic the Kremlin has long seen as part of its privileged sphere of influence.
A failure by Russia to compel the militants to release the hostages could push the U.S. and Europe closer to passing broad economic sanctions against Russia—similar to the crippling measures imposed on Iran— that Western officials have deemed a last resort.
On Sunday, the self-appointed, pro-Russia rebel mayor of Slovyansk, Vyacheslav Ponomaryov, trotted out seven Western military inspectors from Germany, Poland, Denmark, Sweden and the Czech Republic and their translator, all seized late Friday at a makeshift checkpoint in nearby Kramatorsk.
One of the inspectors was later released.
The inspectors are members of their home countries' militaries and part of an inspection team that arrived in Ukraine under an Organization for Security and Cooperation in Europe pact called the Vienna Document, which sets out guidelines for exchanging military information and hosting inspections.
They aren't part of the OSCE special monitoring mission, which is made up of civilians and also operates in southeast Ukraine.
The European hostages gave a news conference in Slovyansk under the watch of armed pro-Russia rebels who have accused the inspectors of being spies for the North Atlantic Treaty Organization (NATO).
The group numbers 13 in total, including five Ukrainian soldiers who were escorting the mission.
The Ukrainians didn't appear at the news conference.
"We have no indication when we will be sent home to our countries and to see our families," Axel Schneider, the German colonel leading the mission, told the news conference.
"The conditions…are not clear to us. It is not us [who] determine the decisions."
Col. Schneider said the European team initially stayed in a basement but was then moved to a place with heat and air conditioning.
He said they were traveling on diplomatic passports, adding that he didn't know the whereabouts of the Ukrainians or anything about their welfare.
Negotiators for the OSCE arrived in Slovyansk and held talks later Sunday with the pro-Russia militants on the release of the team.
The negotiators secured the release of one of the inspectors, a Swedish major, before leaving the city, said a spokesman for the OSCE's special monitoring mission in Ukraine.
An OSCE spokeswoman said he was released due to a medical condition.
German Foreign Minister Frank-Walter Steinmeier called the news conference in Slovyansk "repulsive" and said it is Russia's duty to force the militants to release the hostages.
Another top German official, Gernot Erler, warned that Germany could begin discussing imposing economic sanctions on Russia this week.
Russia's Foreign Ministry on Saturday said it would take steps to try to solve the situation with the European hostages but appeared to blame the Ukrainian government for inviting the inspectors.
The ministry suggested Kiev should have been more careful in deploying inspectors in a place "where the authorities don't control the situation and have launched a military operation against the people of their own country."
Both Russia and Ukraine are OSCE members and signatories of the Vienna Document.
Ukraine's Interior Ministry on Friday said Kiev officials had contacted the pro-Russia militants about the hostage situation.
"They refused to release the hostages, saying they had to agree with the competent authorities of the Russian Federation," the ministry said.
Igor Strelkov, the commander in charge of the Slovyansk militants, said in an interview with a Russian tabloid on Saturday that he would negotiate the release only with the authorities of the Russian Federation.
The pro-Russia militants began taking hostages this month, including American journalist Simon Ostrovsky, who was later released.
They have held other journalists, pro-Kiev activists and people they have accused of being far-right Ukrainian nationalist provocateurs.
Many have been held in a Ukrainian security service building in Slovyansk that the militants seized in early April.
They are also holding the city's elected mayor.
Ukraine's federal security service, known as the SBU, confirmed that three of its officers had been taken hostage late Saturday in the city of Horlivka, southeast of Slovyansk, when they tried to arrest the primary suspect in the murder of local councilman Volodymyr Rybak .
The SBU officers had been trailing the local pro-Russia commander in charge of Horlivka's occupied police station, who Ukrainian authorities named as a suspect last week.
He hasn't responded to the allegations.
Alexei Petrov, a spokesman for the rebel movement there, said the pro-Russia activists realized that the SBU officers were following the local commander and other militants and seized the group in the center of Horlivka.
The SBU officers were later taken to Slovyansk, where Mr. Strelkov presented them in their underwear for questioning to Russian news outlets.
The three men appeared bloodied with tape and gauze over their eyes and their hands tied behind their backs.
In the footage, Mr. Strelkov pointed out that Kiev's attempt to surround Slovyansk and blockade his command hadn't prevented him from transferring the three SBU officers from Horlivka into the city.
"We will wait for a proposal from the Ukrainian side on the exchange of these people for our comrades," Mr. Strelkov said.
The militants have demanded the release of a number of activists who have been arrested during protests and other actions, including Pavel Gubarev, who helped lead the pro-Russia movement in Donetsk.
After declaring himself Donetsk's "people's mayor," Mr. Gubarev was arrested March 6 for a period of two months pending trial on suspicion of threatening Ukraine's territorial integrity and provoking mass disorder.
His wife has called the charges inaccurate.
Russian Foreign Minister Sergei Lavrov has called for Mr. Gubarev's release, pointing out that an agreement struck this month in Geneva by the U.S., Russia, the EU and Ukraine calls for amnesty for such activists.
An OSCE representative met with Mr. Gubarev on Saturday and confirmed he is on a hunger strike, RIA Novosti reported.
Amnesty International, the human-rights organization that advocates for prisoners, has denounced the behavior of the militants.
"Taking hostages and using them as bargaining chips for political gain is as abhorrent as it is unlawful," Heather McGill, a Ukraine researcher for Amnesty, said late last week.
She called on them to release all who are being held unlawfully.
Source: The Wall Street Journal