SLOVYANSK, Ukraine (AP) -- The well-armed, Moscow-backed insurgency sowing chaos in eastern Ukraine scored a new victory Wednesday, seizing armored vehicles and weapons from underequipped government forces, then rolling through two cities to a hero's welcome.
Responding to what it sees as Russia's aggression, NATO announced it was increasing its military presence along its eastern border, closest to Russia and Ukraine. And the Obama administration moved to ratchet up its response, preparing new sanctions on Russia and boosted assistance for the struggling Ukrainian military.
Wednesday's setbacks came just 24 hours after a much-touted Ukrainian army operation to retake control of Solvyansk and other cities in the restive east, and appeared to reflect growing indecisiveness by the new Kiev leadership, which has vowed for days to re-establish its authority there.
With tens of thousands of Russian troops deployed along the border with Ukraine, there are fears the Kremlin might use the instability in the predominantly Russian-speaking region as a pretext for seizing more territory beyond its annexation of Crimea last month.
The day began with throngs of residents in the eastern city of Kramatorsk, some 10 miles (15 kilometers) south of Slovyansk, encircling a column of Ukrainian armored vehicles carrying several dozen troops. Soon after, masked gunmen in combat gear, wearing the black-and-orange St. George ribbons distinguishing them as pro-Russian militia, reached the site.
Without offering resistance, the Ukrainian soldiers surrendered the vehicles to the militiamen, who sat atop them as they drove them into Slovyansk, Russian flags fluttering in the breeze.
They were greeted by a cheering crowd of some 1,000 people that, although numerous, did not necessarily represent the views of the entire city of 130,000.
One Ukrainian soldier said they had defected to the pro-Russian side, but another suggested they were forced at gunpoint to hand over the vehicles.
"How was I supposed to behave if I had guns pointed at me?" the soldier, who did not identify himself, asked a resident.
Hours later, the Ukrainian Defense Ministry broke its silence, acknowledging the seizure of the military hardware and saying the whereabouts of the Ukrainian soldiers was not known. The Interfax news agency quoted an insurgent leader in Slovyansk, Miroslav Rudenko, as saying the soldiers would be offered the chance to join a local militia or leave the region.
Insurgents in Slovyansk have seized the police headquarters and the administration building, demanding broader autonomy for eastern Ukraine and closer ties with Russia.
Similar seizures have occurred in at least 10 other cities in eastern Ukraine -- and the central government says Moscow is fomenting the unrest in a region that was once the support base for ousted President Viktor Yanukovych, who fled to Russia after months of protests over his rejection of closer relations with the European Union in favor of closer ties with Russia.
Despite the frenzied welcome given the pro-Russian militiamen in Slovyansk, opinions were divided. Some residents were happy with the pro-Russian forces that have now taken effective control over the city.
"We will never allow the fascist Kiev authorities to come here," said Andrei Bondar, 32, a Slovyansk resident.
But others, like Tetyana Kustova, a 35-year-old sales clerk, were appalled by the unrest.
"They are pushing us toward Russia," she said. "They are tearing Ukraine into pieces."
Later Wednesday, in Pchyolkino, a town south of Slovyansk, several hundred residents surrounded 14 Ukrainian armored vehicles. Fearing the troops were sent to quell them, the crowd refused to let the vehicles leave despite the pleas of a Ukrainian officer.
They were soon joined by masked pro-Russian gunmen whose sophisticated firearms and battle-readiness have ignited suspicions they, and other militiamen like them, may be troops under direct Russian command.
To end the standoff, leaders in the crowd told their Ukrainian commander, Lt. Colonel Oleksandr Shvets, they would let his 100-strong troops go if they handed over the magazines from their assault rifles. The soldiers removed the magazines, put them in plastic bags and gave them to the pro-Russian militia.
"We are really tired of all of this confusion," said Sgt. Dmytro Mokletsov. "It's really scary giving away the magazines. We have no weapons now. But we were told to, it was an order."
Reflecting the West's concern over the turmoil in Ukraine, German Chancellor Angela Merkel called Russian President Vladimir Putin to discuss the situation and preparations for diplomatic talks Thursday in Geneva on Ukraine.
The Kremlin said Putin told Merkel that "the sharp escalation of the conflict places the country in effect on the verge of a civil war." Merkel's office said she and Putin had "different assessments" of the events in Ukraine.