Pro-Kremlin rebels in Ukraine braced on Wednesday for a renewed military offensive by Kiev as US troops headed to the 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.

Ukrainian acting President Oleksandr Turchynov ordered late on Tuesday 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, Sloviansk and the other towns in the Donetsk and Lugansk regions," said Deputy Prime Minister Vitaly Yarema, according to the Interfax Ukraine news agency.

Calm in flashpoint town

In the eastern town of Sloviansk, a tense flashpoint town near where the two bodies were found, the streets were calm, with locals walking about as usual.

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 Sloviansk are holding two journalists - a US citizen working for the company Vice News, Simon Ostrovksy; and a Ukrainian working for a pro-Kiev outlet, Irma Krat.

Sloviansk's local rebel leader Vyatcheslav Ponomarev told reporters that the US citizen "is not being detained, was not abducted, has not been arrested" and claimed he was "working" in one of the rebel-occupied buildings.

AFP-Reuters