The central government has so far been unable to rein in the insurgents, who it says are being stirred up by paid operatives from Russia and have seized numerous government facilities in at least nine eastern cities to press their demands for broader autonomy and closer ties with Russia. Complicating the political landscape, many local security forces have switched to their side.
The clashes Tuesday came at Kramatorsk airport, just south of the city of Slovyansk, which has come under the increasing control of the pro-Russian gunmen who seized it last weekend.
The precise sequence of events was mired in confusion amid contradictory official claims.
The commander of the Ukrainian operation, Gen. Vasyl Krutov, speaking outside Kramatorsk airport, said his men managed to thwart an attack by fighters in green military uniforms without insignia who tried to storm the facility in the late afternoon. An Associated Press reporter and camera crew heard rounds of gunfire at the time.
After the armed standoff, hundreds of local people surrounded the airport in response to rumors that government troops were planning to launch a military operation on the city of Kramatorsk itself. Some in the crowd attempted to enter the military facility, prompting Ukrainian troops to fire bursts of warning shots.
In an attempt to defuse the situation, Gen. Krutov came out to speak to the angry protesters but was attacked by them. After a tussle in which his hat was knocked to the ground, he managed to take refuge in the airport.
There were conflicting reports of casualties.
Yury Zhadobin, coordinator of a pro-Russian defense force, said two people were slightly injured and were taken to a hospital. Russian media, without sourcing, claimed anywhere from four to 11 casualties at the airport. Ukraine's government said there were no casualties, adding that Ukrainian forces took an unspecified number of militiamen prisoner.
While Krutov spoke of repelling an attack, the new government in Kiev declared that its forces had recaptured the airport from militiamen.
"I just got a call from the Donetsk region: Ukrainian special forces have liberated the airport in the city of Kramatorsk from terrorists," acting President Oleksandr Turchynov told parliament.
"I'm convinced that there will not be any terrorists left soon in Donetsk and other regions and they will find themselves in the dock — this is where they belong."
Hours earlier, Turchynov had announced the start of what what he called "an anti-terrorist operation" against the pro-Russian insurgents.
He gave few details, saying only that it would be conducted in a "responsible and balanced" manner in order to "defend the citizens of Ukraine, to stop terror, stop crime and stop attempts to tear our country into pieces."
In Washington, the Obama administration gave its tacit support to the Ukrainian military action.
While the use of force "is not a preferred option," White House spokesman Jay Carney said, "the Ukrainian government has a responsibility to provide law and order. And these provocations in eastern Ukraine are creating a situation in which the government has to respond."
Russia's Foreign Ministry issued a statement denouncing the Ukrainian military operation, saying it was "criminal to fight with your own people as they speak out for their legal rights." The ministry called on Russia's "international partners" to condemn the new Ukrainian government's actions.
What was clear is that the area bordering Russia is getting increasingly armed and unstable. Russia has tens of thousands of troops stationed along its border with Ukraine, raising fears that Moscow might use the instability in the predominantly Russian-speaking east as a pretext for an invasion.
Earlier Tuesday, at least 14 armored personnel carriers flying Ukrainian flags, helicopters and military trucks were seen some 25 miles (40 kilometers) north of Solvyansk, along with at least seven busloads of government troops in black military fatigues.
"We are awaiting the order to move on Slovyansk," said one soldier, who gave only his first name, Taras. Two of the helicopters loaded with troops were later seen flying toward Slovyansk and witnesses said they delivered several dozen troops at the Kramatorsk military airfield.
Tuesday's events were viewed entirely differently by Ukraine and Russia.
"We have to tell the Ukrainians the truth: The Russian Federation is waging a real war against Ukraine in the east, in the Donetsk and Luhansk regions in particular," Ukraine's ex-prime minister and presidential candidate Yulia Tymoshenko said.
She called on the West to "recognize Russia's aggression against eastern Ukraine as a war."
Ukrainian First Deputy Prime Minister Vitaly Yarema also accused Russia of sending its troops to Ukraine, saying Kiev has "evidence that those people occupying Slovyansk and Kramatorsk right now are servicemen of the Russian 45th Airborne Regiment."
Ukraine's security services identified one of the leaders of the pro-Russian operation in Slovyansk as a Russian foreign intelligence agent, Igor Strelkov, who it said had also coordinated Russian seizures of military facilities in Crimea.
Dmitry Peskov, a spokesman for Russian President Vladimir Putin, dismissed the claims as absurd.
The Kremlin has strongly warned the Kiev government that if it uses military force it could prompt Russia to walk out of an international conference on Ukraine in Geneva on Thursday.
"You can't send in tanks and at the same time hold talks. The use of force would sabotage the opportunity offered by the four-party negotiations in Geneva," Russian Foreign Minister Sergey Lavrov said.
Leonard reported from Donetsk. Maria Danilova and Nataliya Vasilyeva in Kiev, Lynn Berry and Vladimir Isachenkov in Moscow, Julie Pace in Washington, and Christopher Bodeen in Beijing contributed to this report.———