Russia said on Wednesday that it will strike back if its "legitimate interests" in Ukraine are attacked, raising the stakes in the Cold War-like duel with the United States over the former Soviet republic's future.
Girls wrapped in the Ukrainian flag, cross themselves during a mass prayer service in the northeastern Ukrainian city of Kharkiv on April 23, 2014
NATO responded by cautioning against "veiled threats", saying they violated the spirit of an agreement reached in Geneva last week to try to pull the crisis-hit country back from the brink of civil war.
Moscow is insisting that Kiev withdraw forces sent to eastern Ukraine on an "anti-terrorist" mission to dislodge pro-Russian rebels, who have occupied government buildings there.
Russian Foreign Minister Sergei Lavrov told state-controlled RT television that if Russia or its interests are attacked, "we would certainly respond".
"If our interests, our legitimate interests, the interests of Russians have been attacked directly, like they were in South Ossetia for example, I do not see any other way but to respond in accordance with international law," he said, referring to Russia's armoured invasion of Georgia in 2008.
Lavrov also said presidential elections planned for May 25 would be "destructive" for Ukraine in the absence of "common ground" with the country's Russian-speaking south and east.
As the war of words between Moscow and Kiev's pro-EU authorities intensified, Ukraine's interior ministry said special forces had liberated one small eastern town, Svyatogorsk, from separatists.
But AFP found no military units there -- only dumbfounded residents who said they had never been under rebel occupation.
Both Kiev and Washington believe the current crisis is being deliberately fuelled by Russian President Vladimir Putin in a bid to restore former Soviet glory.
The Kremlin has an estimated 40,000 Russian troops poised on Ukraine's eastern border, prompting Washington on Wednesday to start deploying 600 US troops to boost NATO's defences in eastern European states bordering Ukraine.
The first unit of 150 US soldiers arrived in Poland on Wednesday, with the remainder arriving in Estonia, Lithuania and Latvia in the coming days.
- Journalists held -
The detention by the rebels of two journalists -- an American working for VICE News, Simon Ostrovsky, and a Ukrainian activist, Irma Krat -- in Slavyansk have done nothing to ease the mounting tensions.
The rebel leader in the town, Vyacheslav Ponomaryov, called Ostrovsky a "journalist provocateur" and promised "we will free him in due course".
The US State Department said it was "deeply concerned about the reports of a kidnapping" of Ostrovsky and called for Russia to organise his immediate release.
Slavyansk was also the source of gunfire that damaged a Ukrainian military reconnaissance plane on Tuesday, and the site of a crime scene where two bodies were found that Kiev's acting president, Oleksandr Turchynov, said had been "brutally tortured".
One of the two victims was believed to be a local politician and member of Turchynov's party, which the president used as justification to relaunch the military operations against the insurgents.
The spiralling violence -- while the US and Russia trade accusations of inflaming the situation through proxies in Ukraine -- has scuppered a Geneva accord agreed last week between Kiev, Russia and the West that was meant to move Ukraine away from the brink of civil war.
German Foreign Minister Frank-Walter Steinmeier warned during a trip to Moldova that "each day that passes... makes a solution harder and harder to reach".
Washington has also expressed concern about "the lack of positive Russian steps" and threatened further sanctions if no progress is made soon.
But Lavrov, in his interview with RT, accused the US of orchestrating the new Ukrainian offensive, noting that it was announced immediately after a two-day visit from US Vice President Joe Biden to Kiev.
"The Americans are running the show," he said.
Russia has dismissed the threat of further Western sanctions and insists it has the right to protect the Russian-speaking population in Ukraine.
- Gas cut-off threat -
Russia's gas supplies to Ukraine and Europe have become another source of tensions between the sides.
The vice president of Russia's state-owned Gazprom, Alexander Medvedev, at a Paris news conference late on Wednesday said Ukraine's gas debt, which he calculated would be $3.5 billion by the beginning of May, is "intolerable".
Russia's energy ministry is proposing three-way talks with Ukraine and the European Union on the debt issue, to be held on April 28 in Moscow or another city, the Interfax news agency reported.
Putin has warned in a letter to the EU that Moscow could cut gas supplies in a month's time if Ukraine's bill was not paid in full.
Significantly, that cut-off would come just before a May 25 election Ukraine is scheduled to hold to choose a new president -- a poll Biden this week described as "maybe the most important election in Ukrainian history".
The hardening positions and the flurry of threats and counter-threats has many countries concerned, not least ones in the European Union dependent on Russian gas.