Last Updated Apr 23, 2014 7:11 PM EDT
DONETSK, Ukraine -- Russia's foreign minister on Wednesday promised a firm response if its citizens or interests come under attack in Ukraine - a vow that came after Ukraine announced a renewal of its "anti-terror" campaign against those occupying buildings in its troubled east.
Although Foreign Minister Sergey Lavrov did not specifically say Russia would launch a military attack, his comments bolstered wide concern that Russia could use any violence in eastern Ukraine as a pretext for sending in troops. Large contingents of Russian troops - tens of thousands, NATO says - are in place near the Ukrainian border.
"Russian citizens being attacked is an attack against the Russian Federation," Lavrov said in an interview with Kremlin-funded satellite TV channel RT. "If our interests, our legitimate interests, the interests of Russians have been attacked directly, like they were in South Ossetia, I do not see any other way but to respond in full accordance with international law."
Separately, the Russian foreign ministry issued a statement demanding that Ukraine pull its armed forces out of the crisis-ridden region in the east.
It was unclear from the interview what Russia would regard as its interests in Ukraine. President Vladimir Putin previously has said Russia would be justified in protecting ethnic Russians in Ukraine.
Ukraine's acting president on Tuesday ordered resumption of an "anti-terrorist operation" against pro-Russia forces that have seized police stations and government in at least 10 cities and towns in eastern Ukraine. The order came after the bodies of two people allegedly abducted by pro-Russia insurgents were found.
However, there were no reports Wednesday of any actions taken by the Ukrainian military or security services.
CBS News correspondent Holly Williams reports that some fear Ukraine is edging closer to civil war. Despite the government's new offensive, Russian-speaking separatists refuse to leave the buildings they've occupied across eastern Ukraine. They say their government discriminates against them - and are demanding an independent state, or at least more autonomy.
But Ukraine's leaders and the U.S. accuse Russian agents of stirring up unrest - and aiding this insurgency.
Alexander, a college student who is part of the separatists' inner circle, was asked by Williams if he was worried that the Ukrainian military will try and push you out of this building.
"I think it will be a stupid move for Kiev because in this case they will give Russia a reason to send some military forces here," he said.
Still, it's unclear how much capacity Ukraine's interim government in Kiev has against the insurgents.
A previous operation to reclaimed seized buildings showed few results before it was suspended last week following international talks in Geneva that produced an agreement to de-escalate the crisis. Ukrainian forces claimed to have regained control of one small airport, but insurgents also seized armored vehicles and reports said some Ukrainian soldiers had switched sides.
"Security forces are in a state of disorganization and demoralization," said Kiev-based political analyst Volodymyr Fesenko. "Today, most of them don't want to fight for anyone because they don't know who is going to win tomorrow and how all of this will end."
The army is underfunded and poorly equipped after years of corruption and mismanagement under Viktor Yanukovych, the Russia-friendly president who fled the country in February after months of protests.
Ukraine's fragile truce was broken by a gun battle Sunday that killed three separatists - though the details are murky.
In the town of Slavyansk Tuesday, they buried the dead - chanting "glory to the heroes." Slavyansk has been under the control of masked, pro-Russian gunmen since last Wednesday.) They've even appointed their own mayor - a former Soviet soldier. He said Tuesday that they're expecting the town to be stormed by government forces - and are preparing to repel the attack.
Pro-Russia forces admitted Wednesday they are holding an American journalist, saying he was suspected of spying for Ukrainian ultra-nationalists.
Simon Ostrovsky, a journalist for the Brooklyn-based Vice News, has not been seen since early Tuesday in the eastern city of Slovyansk . The fluent Russian-speaker who also holds an Israeli passport has been covering the crisis in Ukraine for weeks and was reporting about the groups of masked gunmen seizing government buildings in one eastern Ukrainian city after another.
The insurgents in the east are defying last week's international agreement in Geneva that called for all sides to disarm militant groups in Ukraine and to vacate public buildings they are occupying.
Members of the nationalist Right Sector movement have occupied two buildings in the capital, Kiev, for months, but Ukraine authorities have said the priority is to get the gunmen in eastern Ukraine to vacate the government buildings they hold.
Stella Khorosheva, a spokeswoman for pro-Russia insurgents in Slovyansk, confirmed that Ostrovsky was being held at a local branch of the Ukrainian security service that gunmen seized more than a week ago.
"He's with us. He's fine," Khorosheva told The Associated Press. She dismissed claims that they were keeping Ostrovsky hostage, saying the insurgents were not seeking to "exchange him for someone."
"(We) need to be careful because this is not the first time we're dealing with spies," Khorosheva said when asked why Ostrovsky was held captive.
She said he is suspected of spying for the Right Sector "and other enemy organizations."
In Washington, State Department spokeswoman Jen Psaki said U.S. authorities are "deeply concerned" about Ostrovsky's detention, which she said violated the agreement between Russia, Ukraine, the European Union and the Unites States reached last week.
"We condemn any such actions, and all recent hostage-takings in eastern Ukraine, which directly violate commitments made in the Geneva joint statement," she said. "We call on Russia to use its influence with these groups to secure the immediate and safe release of all hostages in eastern Ukraine."U.S. Vice President Joe Biden visited Kiev, the capital, on Tuesday to offer support to the beleaguered interim government.
Since November, Ukraine has been engulfed in its biggest political crisis since the 1991 fall of the Soviet Union. Months of anti-government protests in Kiev culminated in President Viktor Yanukovych fleeing to Russia in late February.
Ukraine's acting government has accused Russia of orchestrating the unrest in eastern Ukraine, which it fears Moscow could use as a pretext for an invasion. Last month, Russia annexed Crimea weeks after seizing control of the Black Sea peninsula.