The Ukrainian Government is holding crisis meetings after voters in the regions of Donetsk and Luhansk elected separatist leaders in unrecognised ballots.
President Petro Poroshenko denounced Monday's vote as an “electoral farce” that violated a key element of a peace deal struck with rebels in Minsk in September, which was meant to move towards a solution for the Ukrainian conflict.
The Foreign Office issued a joint statement with the Visegrad Group, representing the Czech Republic, Hungary, Poland and Slovakia, saying it deplored the “illegitimate” elections.
Calling on Russia to refuse to acknowledge the results to contribute to a peaceful solution, it added: “The only elections that have legitimacy in Ukraine are those held under Ukrainian law.”
The White House also condemned the “sham” vote, threatening financial penalties on Russia.
Under the Minsk Protocol, Kiev claims only elections for local officials were permitted and not those aimed to create a separate state or form a union with Russia.
President Poroshenko claims Vladimir Putin’s Government encouraged the ballot, which could create a new cold war in the post-Soviet block and threaten the further loss of territory by Ukraine after the Crimean peninsula was annexed by Russia.
In retaliation, he threatened to scrap a law that would have given increased autonomy to rebel-controlled areas of eastern Ukraine that saw months of intense fighting earlier this year.
The law would also have offered separatist militants in the Donbass region immunity from prosecution.
Alexander Zakharchenko, a 38-year-old former mining electrician, reportedly won the election as head of the self-titled Donetsk People's Republic, which was proclaimed by armed rebels in April, with 79 per cent of the vote.
In a similar vote in Luhansk, Igor Plotnisky won more than 63 per cent of the vote, according to a rebel representative.
A Russian deputy foreign minister, Grigory Karasin, made no mention of formal recognition but said the newly elected leadership in eastern Ukraine had a mandate to negotiate with Kiev.
Ukrainian leaders have so far refused to hold direct talks with the separatists, calling them “terrorists” and “bandits”.
The elections were the latest development in the crisis that started with the ousting of Ukraine's Moscow-backed leader, Viktor Yanukovich, after months of protests in February.
Ukraine crisis: A timeline of the conflict
Ukraine crisis: A timeline of the conflict
1/22 30 November 2013
Public support grows for the “Euromaidan” anti-government protesters in Kiev demonstrating against Yanukovych’s refusal to sign the EU Association Agreement as images of them injured by police crackdown spread.
2/22 20 February 2014
Kiev sees its worst day of violence for almost 70 years as at least 88 people are killed in 48 hours, with uniformed snipers shooting at protesters from rooftops.
3/22 22 February 2014
Yanukovych flees the country after protest leaders and politicians agree to form a new government and hold elections. The imprisoned former Prime Minister, Yulia Tymoshenko, is freed from prison and protesters take control of Presidential administration buildings, including Mr Yanukovych's residence.
Genya Savilov/AFP/Getty Imageses
4/22 27 February 2014
Pro-Russian militias seize government buildings in Crimea and the new Ukrainian government vows to prevent the country breaking up as the Crimean Parliament sets a referendum on secession from Ukraine in May.
Sean Gallup/Getty Images
5/22 16 March 2014
Crimea votes overwhelmingly to secede from Ukraine and join Russia in a ballot condemned by the US and Europe as illegal. Russian troops had moved into the peninsula weeks before after pro-Russian separatists occupied buildings.
6/22 6 April 2014
Pro-Russian rebels seize government buildings in the eastern cities of Donetsk, Luhansk and Kharkiv, calling for a referendum on independence and claiming independent republic. Ukraine authorities regain control of Kharkiv buildings on 8 April after launching an “anti-terror operation” but the rest remain out of their control.
7/22 7 June 2014
Petro Poroshenko is sworn in as Ukraine's president, calling on separatists to lay down their arms and end the fighting and later orders the creation of humanitarian corridors, since violated, to allow civilians to flee war zones.
8/22 27 June 2014
The EU signs an association agreement with Ukraine, along with Georgia and Moldova, eight months after protests over the abandonment of the deal sparked the crisis.
LOUISA GOULIAMAKI/AFP/Getty Images
9/22 17 July 2014
Malaysian Airlines flight MH17 is shot down over eastern Ukraine, killing all 298 people on board. Ukrainian intelligence officials claim it was hit by rebels using a Buk surface-to-air launcher in an apparent accident.
10/22 22 August 2014
A Russian aid convoy of more than 100 lorries enters eastern Ukraine and makes drop in rebel-controlled Luhansk without Government permission, sparking allegations of a “direct violation of international law”.
11/22 29 August 2014
Nato releases satellite images appearing to show Russian soldiers, artillery and armoured vehicles engaged in military operations in eastern Ukraine.
12/22 8 September 2014
Russia warns that it could block flights through its airspace if the EU goes ahead with new sanctions over the ongoing crisis and conflict
13/22 17 September 2014
Despite the cease-fire and a law passed by the Ukrainian parliament on Tuesday granting greater autonomy to rebel-held parts of the east, civilian casualties continued to rise, adding to the estimated 3,000 people killed
14/22 16 November 2014
The fragile ceasefire gives way to an increased wave of military activity as artillery fire continues to rock the eastern Ukraine's pro-Russian rebel bastion of Donetsk
15/22 26 December 2014
A new round of ceasefire talks, scheduled on neutral ground in the Belariusian capital Minsk, are called off
16/22 12 January 2015
Soldiers in Debaltseve were forced to prepare heavy defences around the city; despite a brief respite to the fighting in eastern Ukraine, hostilities in Donetsk resumed at a level not seen since September 2014
17/22 21 January 2015
13 people are killed during shelling of bus in the rebel-held city of Donetsk
18/22 24 January 2015
Ten people were killed after pro-Russian separatists bombarded the east Ukrainian port city of Mariupol
19/22 2 February 2015
There was a dangerous shift in tempo as rebels bolstered troop numbers against government forces
20/22 11 February 2015
European leaders meet in Minsk and agree on a ceasefire in eastern Ukraine beginning on February 14. From left to right: Belarus President Alexander Lukashenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, France's President Francois Hollande and Ukrainian President Petro Poroshenko.
MAXIM MALINOVSKY | AFP | Getty Images
21/22 13 February 2015
Pro-Russian rebels in the city of Gorlivka, in the Donetsk region, fire missiles at Ukrainian forces in Debaltseve. Fighting continued in Debaltseve for a number of days after the Minsk ceasefire began.
ANDREY BORODULIN | AFP | Getty Images
22/22 18 February 2015
Ukrainian soldiers repair the bullet-shattered windshield of their truck as their withdraw from the strategic town of Debaltseve. Following intense shelling from pro-Russian rebels, Ukrainian forces began to leave the town in the early hours of February 18.
Brendan Hoffman | Getty Images
Russia denounced the overthrow as a coup by a “fascist junta” and allegedly backed separatist rebellions that sprang up in Russian-speaking parts of Ukraine.
Government forces made large losses in the subsequent battles, during which Malaysia Airlines flight MH17 was shot down. Nato and Ukraine offered alleged proof of Russian involvement arming rebels with heavy weaponry but it denied sending troops across the border.
More than 4,000 people have been killed in the conflict, which has led to US and European Union sanctions against Russia.
Additional reporting by Reuters