Leading world powers agreed to slap fresh sanctions on Moscow over the Ukraine crisis as soon as Monday, as tensions mounted amid fears of an imminent Russian invasion.

As the West sought to ratchet up diplomatic and economic pressure on the Kremlin, the situation on the ground in eastern Ukraine threatened to escalate, with pro-Russian rebels kidnapping an international team of Organisation for Security and Co-operation in Europe (OSCE) observers, accusing them of being Nato spies.

And Ukrainian Prime Minister Arseniy Yatsenyuk said Russia violated his country’s airspace seven times overnight with an aim “to provoke Ukraine to start a war.” Moscow denied any transgression by its warplanes.

Yatsenyuk cut short a visit to the Vatican City as fears grew that the tens of thousands of Russian troops conducting military drills on the border could soon start an invasion.

A Western diplomat warned: “We no longer exclude a Russian military intervention in Ukraine in the coming days.”

International efforts continued to secure the release of a 13-member mission from the OSCE in Europe held hostage by pro-Russian militants in the flash point city of Slavyansk.

The chief of the insurgents’ self-styled “Republic of Donetskm” 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.

The OSCE observers were sent to Ukraine to monitor the April 17 accord signed in Geneva by Russia, Ukraine, the US and the EU that was meant to de-escalate the crisis.“They were invited by the Ukrainian authorities” and their safety “rests fully with the receiving side,” the foreign ministry in Moscow said.

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,” said the Kiev authorities.

In a joint statement, the G7, consisting of the US, Britain, France, Japan, Canada, Germany and Italy, said it would “move swiftly to impose additional sanctions on Russia.”