Slavyansk, Ukraine (CNN) -- Pro-Russian separatists holding a European military observer team in eastern Ukraine released one of the observers for medical reasons Sunday, shortly after parading them before cameras, officials on both sides of the dispute said.
At least seven of the inspectors from the Organization for Security and Co-operation in Europe appeared at a news conference staged by the self-declared mayor of Slavyansk, Vyacheslav Ponomarev, who referred to them as "prisoners of war."
The freed observer was from Sweden and had been suffering from diabetes, Ponomarev spokeswoman Stella Khorosheva told CNN. And Michael Bociurkiw, an OSCE spokesman in Kiev, the Ukrainian capital, called it "a welcome development."
Holger Schmuck, one of the German members of the team, said earlier that their captors had ensured the ailing observer had all the water and sugar he needed and were taking particular care of him.
The monitors were seized Friday outside Slavyansk, one of the flashpoints in the standoff between Ukraine's interim government and pro-Russian factions challenging its authority in the east. They said that although they have diplomatic status, they went along with Sunday's news conference because the mayor asked them to.
"We are held here; I cannot go home on my free decision ... and it is logical in the eyes of the mayor, Ponomarev, that he can use us to present his positions," delegation leader Axel Schneider said.
Schneider said the team, which includes eight Europeans and several Ukrainians, was being held in the cellar of a building. He said the observers agreed to the news conference so that their families could see them, and that the conditions in which they are being held had significantly improved in the past 24 hours.
The observers are always guarded by armed men, but they had not been threatened, he said.
Nevertheless, an air of unease hung over the men, whose immediate fate remains uncertain. Talks are under way between the separatists and a second OSCE team in Slavyansk that was sent to seek their release, Khorosheva said.
Ponomarev has accused the monitors of being spies for NATO and said he might exchange them for pro-Russian activists held by Kiev, but it is not clear if that would be a basis for any negotiations. No evidence has been produced to support any claim they strayed outside their mission.
One OSCE mission in Ukraine is tasked with helping to implement an international agreement signed April 17 in Switzerland, which called for illegal militia groups to disarm and leave occupied buildings, among other provisions. However, the abducted inspectors were working in the restive eastern Donetsk province under the mandate of a previously agreed OSCE mission.
The interim government in Kiev has said the OSCE inspectors are being held by terrorists. Besides the Germans and the freed Swede, the other monitors are from Denmark, Poland, Bulgaria and the Czech Republic, Russian state media said.
Obama: Unite against Russia's actions
Many eastern Ukraine residents have Russian roots and sympathize with Moscow. But Western nations and Kiev have accused Russia of stirring up unrest in its neighbor and supporting the armed separatists' revolt.
Earlier Sunday, U.S. President Barack Obama urged the world to unite in its disapproval of Russian actions in Ukraine.
"Russia has not lifted a finger to help -- in fact, there's strong evidence that they've been encouraging the kinds of activities that have taken place," Obama said, speaking from Malaysia, where he is on a diplomatic visit.
Rather than going with sanctions alone and making it a United States versus Russia issue, "it's important for us to make sure that we're part of an international coalition in sending that message and Russia is isolated, rather than (the perception that) the U.S. is trying to pull Ukraine out of his orbit," he said.
G7 leaders have said they will impose additional sanctions on Russia over its role in the crisis. Specific EU and U.S. measures are expected to be announced in the coming days.
Russia's Foreign Ministry insisted Saturday it was taking all possible measures to resolve the situation over the abducted OSCE team. In a statement, it added that the security of the observers is the responsibility of the hosting country.
The perilous face-off intensified Saturday when Russian state news complained that Ukraine had mobilized 15,000 troops in the suburbs of Slavyansk "in order to wipe out the city and its residents."
Quoting a Russian Defense Ministry source, RIA Novosti said satellite photos showed the force forming around the city that has become a friction point between the Ukraine military and pro-Russian militants. The Defense Ministry source said the number of Ukrainian troops put the pro-Russian militants at a disadvantage because the latter are "armed only with small amount of pistols and shotguns."
The source said the photos showed about 160 tanks, 230 infantry combat vehicles and armored personnel carriers, mine throwers and multiple-launch rocket systems.
Russian President Vladimir Putin has repeatedly criticized what he says is Kiev's use of force against Ukrainian civilians.
Developments in Ukraine have come at a rapid pace in recent days:
-- On Sunday, separatist leader Denis Pushilin told CNN that a regional TV station in Donetsk had been seized. "The people have gone there. They want to watch Russian channels and are tired of the Ukrainian TV lies," he said.
-- Militants in the town of Gorlivka have captured Ukrainian Security Service officers who were seeking to arrest a Russian citizen suspected of murdering a pro-Kiev lawmaker, the security service said Sunday.
-- Russian military aircraft "crossed and violated" Ukrainian airspace seven times, Ukrainian Prime Minister Arseniy Yatsenyuk told reporters in Rome on Saturday. The Russian Defense Ministry denied the accusation, according to the state news agency ITAR-Tass.
-- Yatsenyuk met with Pope Francis while in Rome on Saturday. The meeting has been seen as a sign of support from the Vatican for his government.
-- Russia, which already had 40,000 troops on its side of the border, started new military drills Thursday after Ukrainian forces said they killed five pro-Russian militants. A day later, Ukraine launched the second stage of an "anti-terrorist operation" against militants in Slavyansk.
Speaking Saturday, Yatsenyuk urged Russia to pull back its security forces and not to support pro-Russian militants in eastern and southern Ukraine. "We urge Russia to leave us alone," he said in televised remarks.
Ukraine's government has promised constitutional reforms and protections for Russian speakers in a bid to ease the tensions in its eastern regions.
CNN's Nick Paton Walsh reported from Slavyansk; Gul Tuysuz reported from Kiev, and Laura Smith-Spark wrote and reported from London. CNN's Ralph Ellis, Andrew Carey, Alex Felton, Bharati Naik, Ben Brumfield and Boriana Milanova contributed to this report, as did journalist Victoria Butenko in Kiev.