Senior officials from the EU, Russia, the US and Ukraine are to meet next week to discuss the worsening situation in Ukraine.It will be the first four-way meeting since the crisis erupted.
EU foreign policy chief Catherine Ashton will join US Secretary of State John Kerry, his Russian counterpart Sergei Lavrov and Ukraine’s Foreign Minister Andriy Deshchytsia.
Russia annexed Crimea in February and has troops massed along the border.
Kiev and the US accuse Moscow of fomenting unrest in the mainly Russian-speaking east of the country as a pretext to possibly seizing more territory – a claim strongly refuted by Russia.
Moscow has so far refused to recognise the new authorities in Kiev following the ousting of pro-Russian President Viktor Yanukovych in February.
The exact time and place of next week’s talks were not made clear although an EU official confirmed they would be in Europe.
A spokeswoman for Baroness Ashton said she “continues the diplomatic efforts aiming at de-escalating the situation in Ukraine. In this context she will meet foreign ministers of the US, Russian Federation and Ukraine next week”.
On Tuesday, Nato warned Russia that further intervention in Ukraine would be a “historic mistake” with grave consequences.
Nato Secretary-General Anders Fogh Rasmussen said Moscow must pull back troops it has massed on the border with eastern Ukraine.
“I urge Russia to step back and not escalate the situation in east Ukraine,” he said.
Russia’s President Vladimir Putin says there is no intention to invade Ukraine but he reserves the right to protect Russian interests there.
He called on Russia to “engage in a genuine dialogue with the Ukrainian authorities”.
Tension rose at the weekend when pro-Russia activists seized regional government buildings in Donetsk, Luhansk and Kharkiv in the east of Ukraine. They barricaded themselves in and raised the Russian flag.
On Tuesday, Ukrainian authorities said they had retaken control of the building in Kharkiv, detaining some 70 people in a bloodless operation.
But in Luhansk, officials accused “radicals” occupying the state security building of placing explosives and holding about 60 people against their will.
Activists in the building denied having explosives or hostages but said they had seized an armoury full of automatic rifles.
In Donetsk, protesters remained inside the regional authority building, calling for a referendum on secession from Ukraine.
Moscow has warned Ukraine that using force to end the protests could lead to civil war.
As a war of words between Russia and the West hotted up on Tuesday, Mr Kerry said Russian special forces and agents had been “the catalyst behind the chaos of the last 24 hours”.
He said the events “could potentially be a contrived pretext for military intervention just as we saw in Crimea”.
On Tuesday, an EU diplomatic source told BBC News that the European Commission was setting up a special “Support Group for Ukraine” to co-ordinate assistance.
The group will consist of several dozen people and its work could be extended to cover fellow ex-Soviet states Georgia and Moldova, the source added.
Mr Yanukovych fled Kiev for Russia after months of street protests triggered by his refusal to sign an association agreement with the EU in favour of closer ties with Russia.
More than 100 people died in the ensuing unrest.
