Russia issued a blunt warning Wednesday it would respond if its interests are attacked in Ukraine, as pro-Kremlin rebels in the restive east of the country braced for a new military offensive by Kiev.
The threat by Russian Foreign Minister Sergei Lavrov, recalling the 2008 war with Georgia over breakaway South Ossetia, came as US troops were headed to region in a show of force after Washington again warned Moscow of new sanctions over the escalating crisis.
"If we are attacked, we would certainly respond," Lavrov told state-controlled RT television.
"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 did not elaborate, but the reference to South Ossetia strongly hinted at the possibility of military action.
The United States, meanwhile, said it plans to deploy 600 troops to Poland and the Baltic states starting Wednesday to "reassure our allies and partners".
Ukraine's acting president Oleksandr Turchynov late Tuesday ordered a new "anti-terrorist" operation against separatists holding a string of eastern towns after the discovery of two "brutally tortured" bodies.
One of the dead was a local politician from Turchynov's party who was kidnapped nearly a week ago, the leader said, blaming his death on the rebels.
Kiev's offensive threatens to sound the final death knell for an already tattered agreement struck last week in Geneva between Ukraine, Russia and the West to ease the crisis, which some fear could tip the country into civil war.
"Security agencies are working to liquidate all the groups currently operating in Kramatorsk, Slavyansk and the other towns in the Donetsk and Lugansk regions," said Deputy Prime Minister Vitaly Yarema, according to the Interfax Ukraine news agency.
Lavrov charged that the timing of the renewed offensive during US Vice President Joe Biden's visit to Kiev on Tuesday demonstrated that "the Americans are running the show".
In the eastern town of Slavyansk, a tense flashpoint town near where the two bodies were found, the streets were calm, with locals walking about as usual.
A handful of rebels wearing camouflage gear and ski masks but with no apparent weapons stood outside the barricaded town hall they are occupying.
In front of the building were displayed three photos of militants who were killed in a weekend attack on a roadblock the separatists have blamed on pro-Kiev ultra-nationalists.
On Tuesday, a Ukrainian reconnaissance plane was hit by small-arms fire from the town, but the aircraft landed safely with none of its crew hurt.
Pro-Moscow insurgents in Slavyansk are holding two journalists, an American working for the company Vice News, Simon Ostrovsky, and a Ukrainian working for a pro-Kiev outlet, Irma Krat.
Slavyansk's local rebel leader Vyatcheslav Ponomarev told reporters that the American "is not being detained, was not abducted, has not been arrested" and claimed he was "working" in one of the rebel-occupied buildings.
However the Twitter feed of the normally prolific journalist has been inactive for a day.
US Secretary of State John Kerry, in an overnight call to Russian Foreign Minister Sergei Lavrov, "expressed deep concern over the lack of positive Russian steps to de-escalate, cited mounting evidence that separatists continue to increase the number of buildings under occupation and take journalists and other civilians captive," a senior State Department official said.
Kerry also warned that a lack of Russian progress on the Geneva deal struck last week would lead to more sanctions on Moscow.
Washington believes Russian President Vladimir Putin is behind the rebellion in the east and the crisis has created a precarious Cold War-style standoff between the Moscow and the West.
The State Department official said Kerry "reiterated that the absence of measurable progress on implementing the Geneva agreement will result in increased sanctions on Russia".
Those messages were underlined on a visit to Kiev on Tuesday by Biden, who also stressed US support for Ukraine's new leaders -- in power since the ouster in February of the pro-Kremlin president after months of pro-EU demonstrations.
Biden called on Russia to pull back its forces from the border, and to reverse its annexation of the strategic Crimea peninsula last month.
Russia has deployed tens of thousands of troops to Ukraine's eastern border, while the United States was sending 600 soldiers to NATO member countries near Ukraine to boost defences in eastern Europe.
A company of 150 troops will arrive in Poland on Wednesday and another 450 are due in Estonia, Latvia and Lithuania in the coming days.
The move sends a "message to Moscow" that "we take our obligations very, very seriously on the continent of Europe," US Rear Admiral John Kirby told a news conference in Washington.
Russia has dismissed the threat of new sanctions and insists that it has the right to protect the Russian-speaking population in Ukraine, a former Soviet republic.
But Prime Minister Dmitry Medvedev has acknowledged his nation's economy was facing an "unprecedented challenge" with recession looming.