SLAVYANSK, Ukraine - The US and Europe are set to slap new sanctions on Russia as early as Monday over the crisis in Ukraine where tensions spiked over the kidnapping of a team of international observers.
The Group of Seven top economies and the European Union signalled Saturday they would step up economic pressure on Moscow early this coming week.
Ukrainian Prime Minister Arseniy Yatsenyuk claimed Russia had violated his country's airspace seven times overnight Friday with an aim "to provoke" Ukraine into starting a war.
US Secretary of State John Kerry told his Russian counterpart Sergei Lavrov that Washington was concerned about "provocative" troop movements along Russia's border with Ukraine and its support for the separatists, which he said "are undermining stability, security and unity in Ukraine".
Yatsenyuk cut short a visit to the Vatican as concern grew that the tens of thousands of Russian troops conducting military drills on the border could soon be ordered to invade.
But Moscow denied any transgression by its warplanes, with Lavrov calling for "urgent measures" to calm the crisis, which has plunged East-West relations to their lowest point since the Cold War.
A Western diplomat warned: "We no longer exclude a Russian military intervention in Ukraine in the coming days."
The diplomatic source noted that Russia's UN envoy, Vitaly Churkin, "has been recalled urgently to Moscow" for consultations.
Meanwhile, international efforts were underway to secure the release of a 13-member mission from the Organisation for Security and Cooperation in Europe held hostage by pro-Russian militants in the flashpoint city of Slavyansk.
The chief of the insurgents' self-styled "Republic of Donetsk", Denis Pushilin, accused them of being "NATO spies" and said they would only be released in a prisoner swap for militants detained by Ukrainian forces.
As indignant Western powers demanded their release, Russia's envoy to the OSCE said Moscow would "take all possible steps in this case".
"We believe that these people should be released as soon as possible," said Andrei Kelin, Russia's envoy.
Russian's foreign ministry added that Moscow was "taking measures" to resolve the situation, but blamed the Ukrainian authorities for the hostage crisis.
"They were invited by the Ukrainian authorities" and their safety "rests fully with the receiving side", the foreign ministry in Moscow said.
The OSCE observers were sent to Ukraine to monitor an April 17 accord signed in Geneva between Russia, Ukraine, the US and EU that was meant to take the heat out of the crisis in the ex-Soviet republic.
An OSCE spokeswoman at the group's Vienna headquarters told AFP there were eight monitors from the mission: four Germans, a Dane, a Pole, a Swede and a Czech.