Russia's Foreign Minister has welcomed an offer by the Ukraine government to hold dialogue with separatists in the east of the country.
Speaking during a visit to Beijing, Sergei Lavrov said the apparent willingness to "resolve through negotiations all the problems relating to the legal demands of the inhabitants of the southeast regions of Ukraine, is certainly a step in the right direction, albeit very belated".
But he warned the use of force against pro-Russian forces in the eastern Ukraine would undermine four-way talks planned for Thursday.
He said "You can't send in tanks and at the same time hold talks, and the use of force would sabotage the opportunity offered by the four-party negotiations in Geneva".
Pro-Russian separatists have seized control of government buildings and set up checkpoints in several cities in eastern Ukraine. Video footage has emerged of a group of separatists stopping a Ukrainian tank and questioning its crew near the village of Rodinskoye.
Interim Ukraine President Oleksander Turchynov said on Tuesday an "anti-terrorist operation" was now under way in the Donetsk region, but insisted it would take place in a "considered" way.
He has accused Russia of harbouring "brutal plans" to destabilise the region by backing separatists and refusing to force them to stand down.
"The plans of the Russian Federation were and remain brutal," he said. "They want not only for Donbass (Donetsk region), but for the whole south and east of Ukraine to be engulfed by fire".
Politicians attacked by activists
Ukraine authorities earlier said pro-Russian separatists in Kramatorsk had voluntarily surrendered the police headquarters but tensions remained high elsewhere in the east.
Meanwhile, two pro-Russian politicians have been attacked by pro-Western activists as tensions remain high.
Oleh Tsaryov, a candidate in the presidential elections on May 25th, was pelted with eggs and beaten as he left a TV studio in Kiev on Tuesday. Mikhaylo Dobkin, another Russian-leaning politician, was sprayed with a green disinfectant on Monday night.
It comes after US President Barack Obama urged his Russian counterpart Vladimir Putin to convince pro-Moscow activists to leave buildings seized in around 10 locations in eastern Ukraine.
The West has accused Russia of being behind the seizures in order to justify expanding its control beyond Crimea, which voted to join the Russian Federation in a referendum last month.
A deadline from Kiev for armed men to lay down their weapons has passed and been ignored by the activists, who have seized buildings in around 10 locations.
European Union Foreign Ministers have also agreed to impose asset freezes and visa bans on more Russians, but stopped short of introducing broader penalties on Russia's economy.
Reporter Ian Woods is in Moscow.
