Russia risks further US sanctions over Ukraine, says Kerry
The US has warned Russia to tone down its aggressive rhetoric on Ukraine, or face further sanctions.
US Secretary of State John Kerry told his Russian counterpart over the phone that Moscow must call for pro-Russian gunmen to lay down their weapons.
Kiev and the West say Moscow commands the gunmen - claims denied by Russia.
The Ukrainian authorities have re-launched an anti-terror operation to take back several towns in the east overrun by pro-Russian militants.
Acting President Olexander Turchynov said he had ordered the operation to restart after two men - including local politician Volodymyr Rybak - were found dead after being "brutally tortured".
"The terrorists who effectively took the whole Donetsk region hostage have now gone too far," Mr Turchynov said.
Kiev's military operation to push gunmen out of public buildings began on 16 April but was suspended over the Easter period.
In other developments:
- Some 150 American troops arrive in Poland, the first part of a 600-strong deployment that Washington says is a show of force for its Nato allies in the region
- Pro-Russian militants in Sloviansk are reportedly holding American journalist Simon Ostrovsky, who works for Vice News.
US officials said Mr Kerry had spoke to Russian Foreign Minister Sergei Lavrov on Tuesday and urged Russia to "tone down escalatory rhetoric [and] engage diplomatically in the east".
The officials said Mr Kerry also demanded that Russia "issue public statements calling for those occupying buildings to disarm and stand down in exchange for amnesty", or face increased sanctions.
US Vice-President Joe Biden met Ukraine's new leaders in Kiev on Tuesday and called on Russia to "stop talking and start acting" to defuse the Ukraine crisis.
The US is to provide an additional $50m (£30m) for political and economic reforms in Ukraine, including $11m to help run the presidential election due on 25 May.
The stand-off in towns across the eastern Donetsk region appears to be intensifying.
Mr Rybak, a local councillor for the Fatherland party, was found dead near Sloviansk on Tuesday.
He had gone missing recently and, according to police, his body was found in a river.
The other man killed has not yet been publicly identified.
"These crimes are being committed with the full support and connivance of the Russian Federation," Mr Turchynov said.
The US had previously said it would impose further sanctions should Russia fail to fulfil commitments laid out in the 17 April Geneva accord.
The deal stipulated an immediate end to violence in eastern Ukraine and called on illegal armed groups to surrender their weapons and leave official buildings.
Russian Prime Minister Dmitry Medvedev told the Russian parliament on Tuesday that Russia would be able to "minimise the consequences" of any further sanctions.
Ukraine has been in turmoil since last November, when Kiev was gripped by protests over whether the country should lean more towards Russia or Europe.