Armed soldiers of the Ukrainian special force stand during a military operation near Debaltseve, in the Donetsk region, on Saturday. (ANATOLII STEPANOV/AFP/Getty Images)
A Ukrainian attack on a suspected convoy of Russian military vehicles sent Moscow a tough new message: Ukraine's military is more confident about protecting its territory, and a full-scale Russian invasion could be prolonged and bloody.
If the convoy is confirmed to be Russian military vehicles, the attack Thursday night would mark the first time in the five-month war in the country's east that Ukrainian and Russian forces clashed directly.
"I can't recall Ukrainians firing on a column coming in from Russia," said Ian Brzezinski, a Deputy Assistant Secretary of Defense for Europe and NATO Policy under President George W. Bush. "This is a military beginning to stand on its own two feet."
Western journalists, including a reporter for The Guardian newspaper in London, reported seeing the column enter Ukraine from Russia through a break in a barbed-wire border. The reporters said they stumbled into it while following a Russian aid convoy headed for the Ukrainian border.
The aid convoy consisted of 262 military trucks, most of them painted white and each manned by three former Russian military personnel, according to a report by National Public Radio. When Ukrainian officials began inspecting the trucks Friday, many of them turned out to be mostly empty, according to the BBC.
On Saturday, those trucks remained idled near the border as complicated procedures, involving documents from the Red Cross about what is aboard the vehicles, dragged on for Ukraine border guards and customs agents to approve the cargo.
In February, Russia seized the Crimean Peninsula from Ukraine without any resistance. Since then, the Ukrainian military has become a significant fighting force, as demonstrated by the attack on the convoy and its recent successes against pro-Russian separatists in several eastern Ukraine cities.
If Russian President Vladimir Putin plans to ramp up the fight with Ukraine, he risks long-term consequences, loss of life and battlefield deaths, Brzezinski said. Ukraine's military is no match for Russia, but it has passion on its side to defend its territory. Even Ukrainians in the east have not embraced the rebellion.
"Russia now knows it's not going to be able to just walk into east Ukraine and seize it like it did in Crimea," he said. "Ukrainians will fight and fight and fight."
Michael McFaul, the U.S. ambassador to Moscow until February, wrote on Twitter that if Ukrainian forces destroyed a column of regular Russian military forces, "then this has escalated in a major way."
Ukrainian President Petro Poroshenko told British Prime Minister David Cameron that "the majority" of the Russian armored column had been destroyed by the Ukrainian artillery" Thursday night, according to a statement from Poroshenko's office.
Russia's defense ministry denied Friday that any military force crossed its border into east Ukraine and called the Ukrainian report "some kind of fantasy."
Damon Wilson, who served as a Russia and Ukraine expert under Presidents George W. Bush and Bill Clinton, says Putin is trying to sow confusion while continuing to deny Western claims that Russia is behind the unrest in eastern Ukraine.
"His rebels are in trouble and he's got to do something," Wilson said.
Ukrainian troops have surrounded and besieged the eastern cities of Donestsk and Luhansk, the last remaining separatist strongholds, where separatist fighters "are trying to burrow into the civilian population," Brzezinski said.
Ukraine has proceeded slowly in part to limit civilian casualties, adopting a "slow strangulation approach" with "the hope that separatists accept that they will lose," he said.
Contributing: The Associated Press