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 the 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 and accusing them of being North Atlantic Treaty Organisation (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 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.”

The diplomatic source noted that Russia’s UN envoy, Vitaly Churkin, “has been recalled urgently to Moscow” for consultations.

Meanwhile, 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 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.

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 de-escalate the crisis in the ex-Soviet republic.

“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 effectively terrorising the whole international community,” said the Kiev authorities.

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

“These sanctions will be coordinated and complementary, but not necessarily identical. US sanctions could come as early as Monday,” a senior US administration official said.

The United States and the European Union have already targeted Putin’s inner circle with visa and asset freezes and imposed sanctions on a key Russian bank.

A senior White House official said the next round could target “individuals with influence on the Russian economy, such as energy and banking“.

While Obama has ruled out sending US or NATO forces into Ukraine, Washington has begun deploying 600 US troops to bolster NATO’s defences in nearby eastern European states.

Ukraine’s Deputy Foreign Minister Danylo Lubkivsky told reporters at the United Nations his country would exercise restraint against pro—Russian separatists to “avoid any victims or casualties”.