American paratroopers arrived in Poland Wednesday as part of a 600-strong deployment to Russia's jittery NATO neighbors in Eastern Europe.

The act is a clear message to Moscow, which responded today, saying it is ready to react militarily if its "interests" are attacked in Ukraine.

"Russian citizens being attacked is an attack against the Russian Federation," Russian Foreign Minister Sergei Lavrov said.

Ukraine says it's relaunching its "anti-terrorism" operation. So far, it has been an unsuccessful attempt to quash unrest sweeping Eastern Ukraine where pro-Russian forces have seized government buildings and even towns.

As tensions rise, so have the number of threats against foreign journalists, including American Simon Ostrovsky, a reporter for Vice News. He's being held by the rebel mayor of the occupied town of Sloviansk.

"Ostrovsky was giving incorrect, one-sided information," Vyacheslav Ponomarev told ABC News, adding that Ostrovsky needs to be taught a lesson.

"He feels fine," Ponomarev said. "We're feeding him well, nothing unpleasant is happening to him."

The mayor said Ostrovsky will eventually be released. The United States is calling on Russia to use its influence on these pro-Russian groups to get him out but so far there is no sign of his ordeal ending any time soon.