Ukrainian forces launched its first major assault on the pro-Russian stronghold in the eastern city of Slovyansk on Friday. (Reuters)

The Kremlin on Friday called a major Ukrainian military effort to clear pro-Russian separatists out of the country’s occupied eastern cities “criminal” and said that President Vladimir Putin was being kept informed about the violence unfolding there as the conflict appeared to enter a dangerous new phase.

With 40,000 Russian troops massed on Ukraine’s border and Kremlin threats to intervene militarily if the interests of ethnic Russians are in peril, it seemed possible that even a small spark could ignite a tinderbox regional conflict. Russia on Friday requested that the U.N. Security Council meet to discuss the situation.

Hours after two Ukrainian military helicopters were shot down early Friday morning, Putin’s spokesman, Dmitry Peskov, told Russian news agencies that the Ukrainian operation to dislodge pro-Russian separatists was a crime and had “effectively destroyed the last hope for the implementation of the Geneva agreements” reached on April 17 and intended to defuse the crisis.

On pro-Kremlin television stations, people described as Russian-speaking residents of eastern Ukraine declared that the Ukrainian military actions against the pro-Russian separatists “meant war.”

“The authorities in the Ukrainian capital should come to their senses and stop the killings of their own citizens. Otherwise, the country’s fate may be extremely sad,” Russian Prime Minister Dmitry Medvedev wrote Friday on his Facebook page.

The Ukrainian Security Service said that one of the helicopters was shot down with a surface-to-air missile, which it suggested meant that the pro-Russian separatists who have taken over cities in eastern Ukraine were receiving outside assistance. Ukrainian and U.S. officials have said that Russian operatives are on the ground in eastern Ukraine helping to organize the anti-Kiev disorder.

The tension is the worst between Russia and the West since the end of the Cold War, with Russia and NATO allies conducting military exercises on opposite sides of the frontiers between Russia and the NATO states of Estonia, Latvia, Lithuania and Poland.

A total of 600 U.S. Army paratroopers have been conducting exercises in those NATO nations, and NATO itself has stepped up its air patrols in the region.

More than 100 British troops were dispatched to Estonia on Friday for exercises of their own, the British Defense Ministry said. They are expected to remain there until May 23. Four NATO minesweepers and a support ship arrived Thursday at the Lithuanian port of Klaipeda for exercises there that are expected to last until the end of May. The port is less than 50 miles from the Russian military enclave of Kaliningrad.

Russia, meanwhile, this week started military helicopter exercises on its border with Latvia and Lithuania.

Ukrainian aviation authorities on Friday banned Russian flights to the hub cities of Kharkiv and Donetsk, the Russian Federal Air Transport Agency said. The move was apparently an attempt to restrict the ability of Russians to travel quickly to the eastern part of Ukraine. Ukrainian and U.S. authorities have accused Russia of sending its own citizens into eastern Ukraine to foment violence.

Putin on Thursday sent a special envoy to eastern Ukraine to negotiate with pro-Russian separatists who have taken seven international observers hostage in Slovyansk. But the mission of envoy Vladimir Lukin was “under threat” Friday, the Russian Foreign Ministry said in a statement, calling for assurances about Lukin’s safety.

The Kremlin said Friday that it had lost contact with Lukin for part of the day, although it later said contact was reestablished.

Lukin, Russia’s human rights commissioner, was also Putin’s special envoy to Kiev in February during negotiations before Ukraine’s then-president, Viktor Yanukovych, fled the country in the face of popular protests.

The Foreign Ministry said that English-speaking foreigners had been seen among the Ukrainian forces mounting the assault on Slovyansk on Friday, echoing its previous charges that U.S. contractors were involved in Ukraine’s response to the unrest in the east.

“The usage of the army against its own people is a crime and will lead Ukraine to disaster,” the Foreign Ministry said.

The White House and State Department have denied previous accusations by Russian authorities that Greystone, a U.S. military contractor, was working alongside the Ukrainian military.

Greystone “does not currently, nor do we have any plans to send personnel to the Ukraine,” company spokeswoman Tiffany Anderson said Friday in a statement.