KIEV, Ukraine (CNN) -- The world must unite to show Russia disapproval of its actions in Ukraine, U.S. President Barack Obama said Sunday.
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, speaking from Malaysia, where he is on a diplomatic visit.
"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, referring to the actions of armed pro-Russian protesters.
A team of European and Ukrainian military observers seized Friday by pro-Russian separatists in Slavyansk remains in their control despite international pressure for the inspectors' release.
At least seven of the inspectors from the Organization for Security and Co-operation in Europe appeared before journalists at a news conference staged Sunday by the self-declared mayor of Slavyansk, Vyacheslav Ponomarev.
The head of the OSCE delegation, which includes eight Europeans and several Ukrainians, said they were being held in the cellar of a building.
No indication was given of when, or if, the team will be released.
The observers said that they have diplomatic status but had given the press conference so their families could see them.
Another OSCE delegation is due to arrive in Slavyansk on Sunday and is expected to discuss ways to free the imprisoned delegation, a spokeswoman for the mayor, Stella Khorosheva, told CNN.
Militants in the town of Gorlivka have also 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.
The perilous face-off intensified Saturday when Russian state news complained that Ukraine had mobilized 15,000 troops in the suburbs of Slavyansk in eastern Ukraine "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." Many eastern Ukraine residents have Russian roots and sympathize with Moscow.
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:
-- 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.
-- 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.
-- G7 leaders said they would impose new sanctions on Russia over its role in the crisis. EU and U.S. measures are expected to be announced in the coming days.
The Ukrainian prime minister 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.
Inspectors seized in Slavyansk
On Saturday, the fate of the kidnapped military inspectors preoccupied world leaders.
The OSCE inspectors were detained as they entered Slavyansk, along with five Ukrainian military representatives and the driver of their bus, Ukraine's Interior Ministry said.
The Ukrainian Security Service said the group was being kept under "inhumane conditions" by the militants.
Ponomarev told reporters Saturday that one of the "prisoners" has diabetes, but has the medicine he needs.
Separatist leader Denis Pushilin, self-declared chairman of the so-called "Donetsk People's Republic," told CNN he doesn't believe they are from the OSCE, but that some are NATO spies.
The German Foreign Office said it had set up an emergency task force to find out what has happened to the team members, four of whom are German. The
others are from Denmark, Poland, Bulgaria and the Czech Republic, Russian state media said.
The OSCE mission in Ukraine is tasked with helping to implement an international agreement signed nine days ago in Switzerland, which called for illegal militia groups to disarm and leave occupied buildings, among other provisions.
CNN's Gul Tuysuz reported from Kiev and Laura Smith-Spark wrote and reported from London. CNN's Andrew Carey and Nick Paton Walsh in Slavyansk and journalist Victoria Butenko in Kiev contributed to this report. CNN's Alex Felton, Bharati Naik, Ben Brumfield and Boriana Milanova also contributed.
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