Ukraine's prime minister says Russian military aircraft have repeatedly crossed into Ukraine's airspace, in what he called Russian aggression designed to undermine global security.
Arseniy Yatsenyuk told reporters Saturday that Russian forces had "violated" Ukrainian airspace seven times overnight.
"We do understand the reason Russian military did it. The only reason is to provoke Ukraine to strike missile and to accuse Ukraine of waging the war to Russia," he said.
On Friday, U.S. military officials also said Russian aircraft had flown into Ukrainian airspace, a charge Russia denies.
In a Saturday statement carried by the Itar Tass news agency, Russia's Defense Ministry said its "objective monitoring of the air situation" had not detected any air border violations.
Prime Minister Yatsenyuk had made the accusation in Rome, after announcing he was cutting short a trip to Italy that included talks with Pope Francis.
The pontiff told Yatsenyuk that he would "do everything possible" to promote peace in Ukraine, where pro-Russian separatists continue to occupy government buildings in about a dozen cities in eastern Ukraine.
According to VOA correspondent Brian Padden, who is on the ground in the eastern Ukrainian city of Donetsk, anti-American sentiment appears to be on the rise among pro-Russian separatists.
An angry mob confronted him on Saturday as he tried to cover a rally in front of an occupied building. He says protesters accused him of supporting a "fascist" U.S. government.
"As we were walking away, the crowd just got more angry and started following us and one guy tried to grab my colleague's camera," he said. "I tried to stop him. Then he grabbed me and another guy came with a baton. But before anything could really deteriorate into a real scuffle, the police kind of came between all of us and pulled us out and we just kept walking."
In another development, Russia vowed to help free a team of international military observers who are being detained by pro-Russian separatists who suspect the observers are "NATO spies."
On Friday, the separatists seized a bus carrying more than a dozen people from the Vienna-based Organization for the Security and Cooperation in Europe (OSCE), near the town of Slovyansk.
According to a senior State Department official, U.S. Secretary of State John Kerry on Saturday demanded full Russian support "without preconditions" in efforts to free the European monitors.
Kerry delivered his demand during a telephone call to his Russian counterpart Sergei Lavrov, who said Ukraine must stop its military operations in the country's southeastern region as part of efforts to end the crisis.
In a statement, Moscow later said it is taking what it called "all measures to resolve the situation," but blamed Ukrainian authorities for failing to secure the safety of the OSCE team.
In a Saturday statement, a White House official said U.S. President Barack Obama underscored the importance of solidarity in responding to Ukraine's crisis during talks with his European counterparts.
Earlier, the Group of Seven major economies announced it had agreed to "move swiftly" on new sanctions against Russia because of its alleged actions in Ukraine.
In a joint statement, the G-7 nations of Canada, Britain, France, Germany, Italy, Japan, and the U.S. said they would take measures to intensify "targeted sanctions" against Moscow.
A U.S. official said the sanctions could begin as early as Monday.
On Saturday, about 150 U.S. troops arrived in Lithuania. They are part of a U.S. contingent of about 600 troops being deployed to the region.
Some information for this report comes from AP.