Ukraine renewed its military offensive against pro-Kremlin separatist activists as US troops headed to region in a show of force against Russia.

Ukraine renewed its military offensive against pro-Kremlin separatist activists as US troops headed to region in a show of force against Russia.

 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.

"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.

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.

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.

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.