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.
Members of Arseniy Yatsenyuk's delegation told journalists he was returning to Ukraine later on Saturday amid escalating Ukrainian-Russian tensions.
The pope was meeting with foreign leaders arriving for the ceremony Sunday that will raise Popes John Paul II and John XXIII to sainthood. Francis gave Yatsenyuk a fountain pen, telling him, "I hope that you write 'peace' with this pen."
Yatsenyuk replied: "I hope so, too."
Russian warplanes violated Ukraine's airspace several times in the past 24 hours, a Pentagon spokesman said Friday, in the latest sign of a mounting confrontation between Moscow and Kiev.
"I can confirm that on several occasions in the last 24 hours, Russian aircraft have entered Ukrainian airspace," said Colonel Steven Warren.
He did not provide more details, including where the incidents occurred or what kind of Russian planes were involved.
Warren urged Russia to take "immediate steps to de-escalate the situation."
With tensions soaring, US defence secretary Chuck Hagel had tried to arrange a phone conversation since Thursday with his Russian counterpart, Sergei Shoigu, to discuss the crisis, but Moscow has yet to respond to the request, Warren said.
The chairman of the US Joint Chiefs of Staff, General Martin Dempsey, had spoken to his military counterpart in Moscow, Warren said.
After Ukraine announced military operations to counter pro-Kremlin rebels, Russia ordered its troops and aircraft massed on the border to launch new exercises.
US defense officials have confirmed a flurry of troop movements associated with the declared drills and acknowledged there is ample Russian firepower on the border to quickly seize territory in eastern Ukraine -- if Moscow chose to act.
Germany meanwhile said pro-Russian separatists had detained 13 members of an OSCE observer mission in Ukraine.
Kiev's deputy foreign minister, Danylo Lubkivsky, said at the United Nations on Friday that his government would exercise restraint its moves against separatists.
But if Russia crossed the border, "we will fight to defend our country," he said.
(With inputs from AP and AFP)