By Bertrand de Saisset
Pro-Kremlin rebels in Ukraine are bracing for a renewed military offensive by Kiev as US troops headed to region in a show of force after Washington again warned Moscow over the escalating crisis.
The United States said it plans to deploy 600 troops to Poland and the Baltic states to "reassure our allies and partners" after threatening Russia with more sanctions.
Ukraine's acting president Oleksandr Turchynov late on Tuesday (local time) 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.
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 Ostrovksy, and a Ukrainian working for a pro-Kiev outlet, Irma Krat.
Slavyank'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, like Kiev, believes Russian President Vladimir Putin is behind the Ukrainian rebellion in the east.
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 US Vice President Joe Biden, who also stressed US support for Ukraine's new leaders.
Biden called on Russia to pull back its forces from the border, and to reverse its annexation of the strategic Crimea peninsula last month.
The deepening of the crisis has given rise to a Cold War-style stand-off between Moscow and the West.