US Secretary of State John Kerry expressed concern to Russian Foreign Minister Sergei Lavrov Saturday about “provocative” Russian troop movements along Ukraine’s border, a US official said.

In a morning call with his counterpart, Kerry also urged full Russian support for efforts to free a mission from the Organization for Security and Cooperation in Europe held hostage by pro-Moscow militants in the flashpoint city of Slavyansk.

The top US diplomat “expressed continued concern that Russia’s provocative troop movements on Ukraine’s border, its support for separatists and its inflammatory rhetoric are undermining stability, security and unity in Ukraine,” the senior State Department official said.

He also “urged Russian support without preconditions for the efforts of the OSCE and the Government of Ukraine to liberate the Vienna Document inspectors and their Ukrainian guides who are being held hostage by pro-Russian separatists in Slovyansk.”

Kerry’s comments came as the Group of Seven top economies and the European Union signaled they would step up economic pressure on Moscow early next week amid fears Russia was preparing an invasion of eastern Ukraine.

Ukrainian Prime Minister Arseniy Yatsenyuk meanwhile claimed Russia violated his country’s airspace seven times overnight with an aim “to provoke” it into starting a war.

But Moscow denied any transgression by its warplanes, with Lavrov calling for “urgent measures” to de-escalate the crisis that has plunged East-West relations to their lowest point since the Cold War.

The United States and Europe have prepared fresh sanctions on Russia they could activate as soon as Monday over the crisis in Ukraine.

The Group of Seven top economies and the European Union signaled they would step up economic pressure on Moscow early next week amid fears Russia was preparing an invasion of eastern 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.

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 Organization 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 immediate 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,” Andrei Kelin said.

Russian’s foreign ministry also said 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 United States and the European Union 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.

This group of eight monitors was accompanied by five Ukrainian army personnel, the defence ministry in Kiev said in a statement.

Speaking to reporters in Rome, Yatsenyuk said the detention was “unbelievable and unacceptable”.

“This is another proof and evidence that these so-called peaceful protesters with Russian ideas are terrorists,” he said.

Ukraine’s own secret services said one of those detained “urgently” needed medical help.

The hostages were being held in “inhuman conditions” and were likely to be used as “a human shield effectively terrorizing the whole international community”, said the Kiev authorities.