Russia's foreign minister has 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 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.
Mr Lavrov said an interview with Kremlin-funded satellite TV channel RT: "Russian citizens being attacked is an attack against the Russian Federation.
"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."
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 yesterday 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.
But there were no reports Wednesday of any actions taken by the Ukrainian military or security services.
Pro-Russia forces admitted today 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 yesterday 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 Mr 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," Ms 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".
When asked why Mr Ostrovsky was held captive, she said: "(We) need to be careful because this is not the first time we're dealing with spies."
She said he is suspected of spying for the Right Sector "and other enemy organisations".
In Washington, State Department spokeswoman Jen Psaki said US authorities are "deeply concerned" about Mr 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."
US vice president Joe Biden visited Kiev, the capital, yesterday 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 then-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.