David Cameron and Barack Obama say the EU and United States "should stand united in their condemnation of Russia's actions" in Ukraine.

In a phone call the Prime Minister and US president have agreed the current situation is "unacceptable".

Russian authorities have yet to withdraw their troops to their bases.

Meanwhile - NATO secretary general, Anders Fogh Rasmussen has announced a full review of its co-operation with Russia.

His comments come after the UN envoy to Ukraine was threatened by unidentified gunmen in Crimea earlier today.

The UN Deputy Secretary General Jan Eliasson says he was accosted by gunmen outside a naval headquarters, and warned 'he should leave Crimea'.

A government building in eastern Ukraine - which was cleared of pro-Russian protesters earlier - has been recaptured by them.

Sky reporter Hannah Thomas Peter - who's outside the UN building in New York says the incident is seen as 'extremely serious':

The Crimean peninsula has come under near complete control by pro-Moscow forces although Russian President Vladimir Putin insists there are no Russian troops there.

Yesterday President Putin suggested the soldiers could have bought their Russian uniforms in a souvenir store.

Speaking at the Oireachtas Foreign Affairs Committee today, the Ambassador of Ukraine to Ireland Sergii Reva said those comments aren't credible:

France, the US and Russia have been holding talks to try to resolve the crisis in Paris.

Former UN Deputy Secretary-General Lord Malloch-Brown doesn't think Russia will invade much further into Ukraine.

Next Wednesday Russian Ambassador Maxim Peshkov will appear before the same Committee.

The appearance of the Ambassador before the Committee follows his meeting yesterday with the Foreign Affairs Minister Eamon Gilmore, who conveyed to him Ireland's condemnation of Russia's actions in Crimea.

Roland Oliphant is a Daily Telegraph journalist in Crimea. He told Newstalk Breakfast ealier it looks like Crimea will remain part of Ukraine with confederated status.Meanwhile, Moscow has said it will do "everything to prevent bloodshed" in Ukraine, despite reports of confrontations between forces loyal to Kiev and pro-Russian armed groups.

Russian Foreign Minister Sergei Lavrov struck the conciliatory tone before he was due to meet face-to-face with US Secretary of State John Kerry for the first time since the stalemate began.

Speaking at a news briefing with his Spanish counterpart Jose Manuel Garcia-Margallo in Madrid, he said it was important for Russia to continue talks with Nato and EU leaders.

Mr. Lavrov also said Moscow would like to see a de-escalation in tensions, sparked by Russian intervention in Ukraine's Crimea region.

But he added that "nobody has the right to be angry with Russia".

Earlier, the EU said it could vote on Russian sanctions on Thursday if there is no de-escalation in the stand-off before then. Russian President Vladimir Putin has previously said any Western sanctions against Moscow would be counter-productive.

America has already threatened sanctions, with a senior US official saying Washington is ready to impose them in days rather than weeks.

Speaking in Paris earlier today, US Secretary of State John Kerry held meetings with Russian, Ukrainian, British and French foreign ministers.

Following these talks Mr Kerry renewed his call for Russia to call troops back from Crimea:

US Secretary of State John Kerry & Russian Foreign Minister Sergei Lavrov

US President Barack Obama spoke to German Chancellor Angela Merkel about a plan to end the stalemate. Under the proposal, Russia would pull back its forces in Crimea to their bases in the peninsula and limit troop numbers to a Ukraine-agreed maximum of 11,000.

However, Mr. Lavrov said that pro-Russian armed groups operating in Crimea were "self-defence" forces who do not answer to Moscow. He added that Russian naval personnel in the region were in their normal positions.

A senior American official has said the plan would also see international monitors allowed in to ensure the human rights of ethnic Russians are protected.

15 countries from the Organisation for Security and Co-operation in Europe (OSCE) have reportedly already agreed to send observers.

Seen as an effort to offer Russian President Vladimir Putin a way out of the crisis without losing face, the plan would pave the way for direct talks between Moscow and the new Ukraine government with the potential for some international mediation.

The proposal would also see planned elections in Ukraine this May go ahead.

But Mr. Obama sounded a cautious note when commenting on the crisis at a fundraiser on Tuesday night.

"We may be able to de-escalate over the next several days and weeks" he said. "It's a serious situation and we are spending a lot of time on it".

The US official added that Mr. Obama had made clear to Ms. Merkel that he would not attend a G8 summit scheduled for June in Sochi, Russia, if the situation in Ukraine had not changed.

Preparatory meetings about the summit have already been suspended.