US President Barack Obama has urged his Russian counterpart Vladimir Putin to use his influence to make separatists in eastern Ukraine stand down.
The phone call between the two leaders came as pro-Russian activists continued to occupy buildings in eastern towns.
For his part, Mr Putin rejected accusations of Russian interference, calling the reports “unreliable”.
EU foreign ministers said on Monday they would expand a list of names targeted by sanctions.
Tension has been steadily rising since Russia annexed the Crimean peninsula, formerly part of Ukraine, last month.
The move, condemned as illegal by Kiev and the West, followed the ousting of Ukraine’s pro-Moscow President Viktor Yanukovych in February.
In other developments
- Unconfirmed reports on Russian media, quoting separatists, speak of Ukrainian armour being on the move near the flashpoint towns of Sloviansk and Kramatorsk
- At least one separatist checkpoint near Sloviansk reportedly came under fire overnight from unknown gunmen in a car but nobody was apparently hurt
- A presidential candidate from eastern Ukraine, Oleh Tsarev, regarded as pro-Russian, appeared before reporters in Kiev with a black eye after being assaulted by protesters as he left a TV studio
‘Grave concern’
The White House said the “frank and direct” conversation between the two presidents was made at Russia’s request.
“The president expressed grave concern about Russian government support for the actions of armed, pro-Russian separatists who threaten to undermine and destabilise the government of Ukraine,” a statementsaid.
“The president emphasised that all irregular forces in the country need to lay down their arms, and he urged President Putin to use his influence with these armed, pro-Russian groups to convince them to depart the buildings they have seized.”
The statement also threatened Moscow with wider sanctions, saying “the costs Russia already has incurred will increase if those actions persist”.
The Kremlin said in a statement that recent unrest in Ukraine’s south-east was “the result of the unwillingness and inability of the leadership in Kiev to take into account the interests of Russia and the Russian-speaking population”.
It said Mr Putin had urged Mr Obama to “use the resources at the disposal of the American side” to help prevent any bloodshed.
It dismissed claims that Russia was interfering in Ukraine, saying the accusations were based on “questionable information”.
UK Foreign Secretary William Hague said on Tuesday the situation in eastern Ukraine was “very dangerous”. Speaking to the BBC, he commended the Ukrainian government for acting “very responsibly throughout this crisis”.
Thousands of Russian troops are reported to be deployed along the border between Ukraine and Russia. Kiev fears any crackdown on pro-Russian groups could trigger an invasion.
Ukraine’s acting President Olexander Turchynov said on Monday an “anti-terrorist operation” would be launched against gunmen occupying government buildings, but a deadline passed with no large-scale action from Kiev.
Rebels appeal to Putin
Pro-Russian rebels have seized buildings in about 10 towns and cities across eastern provinces that form the heartland of Ukraine’s heavy industry.
The groups are demanding referendums on either greater autonomy or an option to join the Russian Federation.
Mr Turchynov suggested Kiev would be open to moving from a republic into a federation and giving broader rights to Ukraine’s Russian-speakers.
But he appeared to suggest a national vote as opposed to regional ones.
In Sloviansk, where armed men seized a police station and other government buildings on 12 April, rebels called for President Putin’s help.
The Kremlin said the Russian president was listening.
Spokesman Dmitry Peskov said: “Unfortunately, there are a great many such appeals coming from the eastern Ukrainian regions addressed directly to Putin to intervene in this or that form.
“The president is watching developments in eastern Ukraine with great concern.”
A four-way meeting on the crisis, involving the EU, Russia, the US and Ukraine is due to be held in Geneva on Thursday.
Source: BBC