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."
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.
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 , near the town of Slovyansk.
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 (the U.S., Canada, Britain, France, Germany, Italy and Japan) 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.