Pro-Russian separatists in eastern Ukraine offered on Saturday to release eight captive international observers in a prisoner exchange.
Eight monitors from the Organisation for Security and Cooperation in Europe (OSCE) were kidnapped in Slaviansk on Friday.
They are from Germany, Sweden, Denmark, Poland and the Czech Republic.
The separatists said they suspected the observers of spying; Ukraine said they were being used as human shields.
"They were soldiers on our territory without our permission. Of course they are prisoners," said Slaviansk de facto mayor Vyacheslav Ponomaryov.
He said the separatists were ready to exchange the monitors for fellow rebels held by Kiev.
"Prisoners have always been coins to exchange during times of war. It's an international practice," he said.
There are reports from Germany that the OSCE has dispatched a negotiating team to try to secure their release.
''We believe that these people should be released as soon as possible," Andrei Kelin, Russia's envoy to the OSCE, was quoted as saying.
"As an OSCE member, Russia will take all possible steps in this case," he said.
Rebel militia continue to occupy official buildings in a dozen eastern cities, defying the Ukrainian government in Kiev.
The West accuses Russia of leading a secessionist revolt in Ukraine's east, since it annexed Crimea. Moscow denies this.
EU diplomats will meet on Monday to discuss new sanctions against Russia. Earlier, the G7 group countries agreed to intensify their sanctions.
The United States and Kiev say Russian jets have violated Ukraine airspace. However, Russia on Saturday rejected the accusations.
"Russia's airspace monitoring systems have not registered any violations of air borders of the states adjacent to Russia, including Ukraine," a defence ministry statement said.
But Prime Minister Arseny Yatseniuk said Russian military aircraft entered Ukrainian airspace seven times overnight.
"The only reason is to provoke Ukraine ... and to accuse Ukraine of waging war against Russia," the prime minister said.
US personnel arrive
Washington deployed 150 paratroopers to Lithuania on Saturday. A total of 600 US troops have now arrived in Poland and the former Soviet Baltic states.
"As threats emerged, we see who our real friends are," Lithuanian President Dalia Grybauskaite said as she greeted the troops at the Siauliai air base.
Without mentioning Russia, she said the presence of US troops would "repel those who encroach on stability in Europe and peace in the region".
"The numbers are not important. If just one of our guests is harmed, this would mean an open confrontation, not with Lithuania but with the United States of America."