Russian tanks and troops fired their way into eastern Ukraine on Thursday and seized a strategic gateway town on the road to the heavily militarized Crimean peninsula in a brazen display of support for pro-Russia separatists fighting Ukrainian government troops.
Defense officials in Kiev, the Ukrainian capital, said two armored Russian columns burst across the border into southern Ukraine at midday and rolled over the town of Novoazovsk on the Sea of Azov, opening a new front in the 5-month-old separatist battle.
The government's account was bolstered by satellite photos released by the North Atlantic Treaty Organization showing convoys of tanks and armored vehicles entering Ukraine from Russia's Rostov region over the last two weeks.
Also exposing Russia's hand in the Ukraine crisis, a key separatist leader said in an interview with Russian state television that at least 3,000 Russian citizens, most of them military veterans or active-duty soldiers on leave, have been fighting in Ukraine on the side of the insurgents.
"They are fighting with us, understanding that it is their duty," Alexander Zakharchenko, the leader of the self-declared Donetsk People's Republic, told Russian state television in an interview aired Thursday.
The evidence of Russian troops and hardware on the territory of sovereign Ukraine undermined Russian President Vladimir Putin's steadfast denials that Russia has been arming and instigating the anti-Kiev uprising, which has taken more than 2,000 lives since April.
President Obama deemed the Russian military intrusion into Ukraine proof, "if there was ever any doubt, that Russia is responsible for the violence in Ukraine."
"This is not a homegrown, indigenous uprising in eastern Ukraine," Obama told a White House news briefing. "The separatists are backed, trained, armed, financed by Russia."
He said Russia was now "more isolated than at any time since the Cold War" and that the latest provocation would lead to a toughening of sanctions, which have already spooked investors who are pulling billions out of the Russian economy.
French President Francois Hollande said the presence of Russian troops in eastern Ukraine was unacceptable. "If the escalation continues, European Union sanctions will remain and could even be strengthened," he warned.
At the daily Ukrainian security briefing in Kiev, officials reported a major expansion of the armored incursion into Novoazovsk, which began Wednesday. Col. Andriy Lysenko of the National Security and Defense Council said the armored columns had crossed into Ukraine after firing rockets over the border at 11 a.m. The badly outnumbered and outgunned ranks of Ukrainian troops in the area were ordered to stand down, the security official said, leaving the town of Novoazovsk for the Russian forces to overrun.
The invasion prompted Ukrainian President Petro Poroshenko to cancel a trip to Turkey and summon the security council for an emergency meeting to deal with the "sharp aggravation of the situation in Donetsk region."
"Today the president's place is in Kiev," Poroshenko said of the latest aggression by its former Soviet sister state.
A report from the security council meeting said the incursion was carried out by regular Russian military forces replacing local militants in Ukraine's Donetsk and Luhansk regions, who lost most of their seized territory last month to a Ukrainian counteroffensive.
After the incursion into Novoazovsk, an area of the Donetsk region that had so far been spared fighting, Ukrainian troops were reinforcing their positions on the seaside road leading to Mariupol, a city of nearly 500,000 that is a key shipping terminus and steel-making venue. The road leads eventually to the Crimean peninsula, where Russia's Black Sea fleet and about 25,000 troops are based.
The Russian deployments onto the Sea of Azov road have heightened fear that the Kremlin is on a mission to seize the corridor to provide a land bridge from the Russian mainland to Crimea.
At NATO's military headquarters in Mons, Belgium, a Dutch general in charge of the alliance crisis management center released the satellite images and said they confirmed Russia's hand in the Ukraine violence.
"Over the past two weeks we have noted a significant escalation in both the level and sophistication of Russia's military interference in Ukraine," said Brig. Gen. Nico Tak.
"Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favoring the Ukrainian military," Tak said at a news conference.
He also said NATO estimates that there are at least 1,000 Russian military personnel directly engaged in fighting in Ukraine and that 20,000 battle-ready troops and hundreds of tanks and armored vehicles are amassed in Russia just across the border.
Tak said the Russian action was aimed at freezing the conflict and confronting Ukraine with a permanent security crisis.
"It's likely that the situation will end in a stalemate," he said. "The foothold that has been created will be expanded and secured so that the separatists will not suffer a defeat."
Ukrainian government forces last month recovered more than half the territory seized by separatists in early spring, a rebellion inspired by the Kremlin's invasion and annexation of the Crimea region.
Putin has so far ignored appeals by the separatists to annex their territory to Russia.
Special correspondent Butenko reported from Kiev and staff writer Williams from Los Angeles. Special correspondent Isabel Gorst in Moscow also contributed to this report.