Ukraine PM Says Russia Violations 'War Provocation' as Rebels Accuse Detained OSCE Team of Being 'NATO Spies'

Ukrainian Prime Minister Arseniy Yatsenyuk said Saturday that Russian military aircraft had crossed the country's airspace seven times "to provoke Ukraine to start a war."

"Russian military aircraft today overnight crossed and violated Ukrainian airspace seven times. The only reason is to provoke Ukraine to start a war," he told journalists at a briefing in Rome, following a meeting with Pope Francis and Italian Prime Minister Matteo Renzi.

However, shortly after the Ukraine PM's statement, Russia's defense ministry denied claims by the Pentagon and Yatsenyuk that its planes had repeatedly violated Ukrainian airspace in recent days.

"Russia's airspace monitoring systems have not registered any violations of air borders of the states adjacent to Russia, including Ukraine," the defense ministry said in a statement carried by the state ITAR TASS news agency.

The Pentagon had said Russian warplanes had violated Ukraine's airspace several times on Thursday.

The Ukrainian PM also said the seizure of 13 international OSCE observers by pro-Kremlin rebels who accuse them of being "NATO spies" was "another proof and evidence that these so-called peaceful protesters with Russian ideas are terrorists."

Russia pledged Saturday to help free the observers, who 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 dangerous crisis in the ex-Soviet republic.

But pro-Russian rebels holding the OSCE observers accused them of being "NATO spies" and vowed to continue detaining them.

"Yesterday, we arrested some NATO spies... they will be exchanged for our own prisoners. I don't see any other way they will be freed," Denis Pushilin, the head of the insurgents' self-declared Donetsk Republic, told reporters.

Pushilin was speaking in front of the SBU security services building in rebel-held Slavyansk, where the OSCE team was being held.

The town's self-styled mayor, Vyacheslav Ponomaryov, also told Russian TV news crews that the OSCE members were being considered "intelligence officers of NATO country members".

"Military personnel from Denmark, Germany, Poland, the Czech Republic, Bulgaria I think and -- from somewhere else, I can't immediately recall -- have been detained," he said in broadcasts seen in Moscow.

"We believe an OSCE mission does not imply the participation of military personnel entering our territory unimpeded and studying our facilities."

Slavyansk has become the epicenter of tensions between pro-Russian protesters and Ukrainian authorities in the eastern part of the country where pro-Kremlin rebels have seized a string of towns.

Late Friday, German Defense Minister Ursula von der Leyen confirmed that pro-Russian separatists had arrested 13 mission members, including the observers, their interpreter and driver. Four of the team are Germans, including three members of the German military.

Swedish Foreign Minister Carl Bildt said on his Twitter feed that one of the OSCE members was a Swede.

Washington called for the immediate release of the OSCE team and State Department spokeswoman Jen Psaki insisted "there is a strong connection between Russia and these separatists". 

The interior ministry in Kiev said that the OSCE observers were stopped at a rebel-held checkpoint as they were entering Slavyansk on Friday and were taken to the SBU building.

The Organization for Security and Cooperation in Europe told Agence France Presse in Vienna, where it is based, that all the observers in its main mission on the ground in Ukraine were accounted for.

However, the detained group appears to be part of a separate, smaller unarmed military verification mission under German command.

The Ukrainian premier -- who is cutting short his trip will miss the canonization of John Paul II and John XXIII because of rising tensions in the eastern part of the ex-Soviet country -- said "Russian aggression aims to undermine global stability."

"We urge Russia to pull back its security forces. We urge Russia to leave us alone," he said, amid fears that Russia could be about to invade.

Yatsenyuk met Pope Francis for a private audience on Saturday, and said he had "asked his Holiness to pray for my country and peace in Europe."

At an exchange of gifts, Yatsenyuk presented Francis with a photograph of Maidan square in Kiev on New Year's night.

"This is where Ukrainians fought for their freedom and rights. Millions of people," he said.

The pope in return gave the Ukraine leader a pen, saying "I hope this pen will sign the peace", to which Yatsenyuk replied "I hope so."

As the two leaders parted, Francis put his hand on his chest and said "I will do everything possible" for peace.

The Group of Seven rich countries have agreed to slap new sanctions on Moscow as early as Monday. Russia has warned it has a "right" to invade to protect Ukraine's Russian-speaking population concentrated in the east and southeast.

In a related matter, several EU sources said Saturday that senior European Union diplomats will meet in Brussels on Monday to consider a fresh round of sanctions against Russia over the crisis in Ukraine.

Ambassadors from the 28 member states "will meet on Monday 28 April with a view to the adoption of an additional list of 'stage 2' sanctions," such as asset freezes and travel bans, said an EU official speaking on condition of anonymity.

A list adding 15 people to the 55 Russians and Ukrainians already blacklisted by the EU was approved in principle on the eve of the April 17 Geneva talks to de-escalate the conflict, an EU diplomat said.

But the EU refrained from putting the sanctions into effect so as not to scuttle the peace talks.

However with tension mounting in Ukraine the last days amid fears Russia might invade, the Group of Seven rich countries Saturday agreed to slap further sanctions on Moscow.

"In line with the G7 statement issued today, we will work to have it all adopted before the end of the day," said the EU official, referring to the list.

Another EU diplomat close to the matter said ministers from the member states would have to be called to Brussels should the 28-nation bloc intend to ratchet up the pressure on Moscow with wide-ranging economic sanctions such as trade embargoes, known as "Phase 3" measures.

"No meeting of ministers has been scheduled for now," the source said. "We will see on Monday."

Meanwhile, the head of the self-proclaimed Donetsk Republic vowed to go ahead with a referendum on independence for the separatist region in eastern Ukraine next month.

Denis Pushilin told reporters that a popular vote on a "declaration of independence" would be held on May 11, despite major doubts over the logistics and legitimacy of such a referendum.

The question asked would be "Do you approve of the declaration of independence of the People's Republic of Donetsk?" said Pushilin.

After the referendum, there would be elections to decide who would lead the region, he said.

"Then, our People's Republic of Donetsk can start to function again," he said.

"We will see what the best deal is for our relations with the rest of Ukraine -- whether we have a federation, a confederation or independence."

He described the referendum as "inevitable" and said there was "all the money needed to organize it".

Political analysts have voiced doubts over whether the self-proclaimed leaders of the separatist region have the resources -- especially financial -- to mount such a referendum.

Unlike March's disputed referendum in Crimea, in which nearly 97 percent of voters opted for Kremlin rule, pro-Russian protesters in Ukraine's east are not seen as well enough equipped to organize a large popular vote.