"There's been an uptick to Russian air activity in the region" to include a flight by two Russian "Bear" bombers off Britain's southern coast Wednesday, said Army Col. Steve Warren, a Pentagon spokesman.
Warren also said "we have continued to see a large volume of equipment flowing from Russia into Ukraine" in violation of ceasefire agreements signed last week in Minsk, the capital of Belarus.
"Russia's actions have been destabilizing throughout the region," Warren said, but Pentagon officials added that there were no immediate plans to change U.S. policy against supplying weapons to Ukraine despite repeated pleas from the Kiev government.
Fighting continued in eastern Ukraine Thursday a day after the rebels forced thousands of Ukrainian troops to flee the rail hub of Debaltseve in defiance of the ceasefire brokered by France and Germany, Reuters reported.
The abandonment of the town marked the worst defeat for the Kiev government in 10 months of fighting in which an estimated 5,000 people have been killed.
However, NATO Secretary General Jens Stoltenberg said Wednesday that "the Minsk agreement is the best opportunity for a lasting peaceful solution" even as the separatists refused to abide by it.
"Russian forces, artillery and air defense units as well as command and control elements are still active in Ukraine," Stoltenberg said, "and there has been a steady buildup of tanks and armored vehicles across the border from Russia to Ukraine."
In addition to the aggression against Ukraine, Russian President Vladimir Putin also posed a "‘real and present danger" to the Baltic states of Lithuania, Lativia and Estonia, said British Defense Minister Michael Fallon.
In comments to the Times and Daily Telegraph newspapers, Fallon said said Putin could launch a campaign of undercover tactics to destabilise the three former Soviet republics who are now NATO members.
"I'm worried about his (Putin's) pressure on the Baltics, the way he is testing NATO," Fallon said.
Lithuanian Foreign Minister Linas Linkevicius told BBC radio that Russia also posed a threat to the wider region, including Moldova.
"Russia is behaving aggressively now as we speak," Linkevicius said, and the threat was not just to the Baltic states "but also with regard to others and NATO has to respond and be ready for these new threats."
Earlier, a British Ministry of Defense spokesman said that Royal Air Force Typhoon fighters scrambled Wednesday to intercept the two Russian bombers which were flying off the southern Cornwall coast.
"The Russian planes were escorted by the RAF until they were out of the UK area of interest," the spokesman said. "At no time did the Russian military aircraft cross into UK sovereign airspace."
-- Richard Sisk can be reached at firstname.lastname@example.org