Ukraine's president is to chair a crisis meeting of his security chiefs, following elections in rebel-held areas which Kiev describes as a "farce".
Months of fighting have devastated the Donetsk and Luhansk regions
In a TV address, Petro Poroshenko said Sunday's polls in parts of the eastern Donetsk and Luhansk regions could derail the "entire peace process".
Two pro-Russian leaders were declared the winners of the polls and are expected to be sworn in on Tuesday.
The West says the elections were illegal, but Russia is backing them.
More than 4,000 people have been killed in fighting in eastern Ukraine since April.
Ukraine accuses Russia of arming the rebels and sending Russian regular troops across the border - a claim denied by Moscow.
A nominal ceasefire has been in place in eastern Ukraine since 5 September, but there have been repeated violations by both sides.
The clashes in the east began a month after Russia annexed Ukraine's southern Crimea peninsula.
'Sham elections'
Addressing the nation on Monday, President Poroshenko said the 2 November elections in Donetsk and Luhansk were "a farce at gunpoint", which had nothing to do with the expression of the voters' will.
Mr Poroshenko said the polls were "a gross violation" of September's Minsk agreement - a roadmap to a peaceful settlement agreed by Ukraine, Russia, the Organization for Security and Co-operation in Europe (OSCE) and also rebel delegations.
He added that he would hold a meeting of Ukraine's Security and Defence Council on Tuesday to propose abolishing a law granting special self-government to rebel-held areas, something which had been approved by Kiev as part of the peace process.
Ukraine and the West had always insisted that the rebel territories must abide by the Minsk deal and hold local elections under Ukrainian law in December.
Late on Monday, the US said it "condemns the illegitimate, so-called 'elections' held on Sunday by Russia-backed separatists in parts of Donetsk and Lugansk".
In a statement, White House National Security Council spokeswoman Bernadette Meehan added that Washington was "concerned by a Russian Foreign Ministry statement that seeks to legitimise these sham 'elections'".
Sanctions warning
Russia earlier said it "respects the will of the people of the south-east". It also called for a "sustainable dialogue" between the authorities in Kiev and the rebels.
Meanwhile, EU foreign policy chief Federica Mogherini said the polls were "a new obstacle on the path towards peace", while Germany said Russia's backing of the vote was "incomprehensible".
Germany has described as "incomprehensible" Russia's backing of controversial Sunday's elections in rebel-held eastern areas of Ukraine.
German Chancellor Angela Merkel's spokesman, Steffen Seibert, warned that Russia could face further sanctions as a result of its position.
The US and EU have already targeted key sectors of the Russian economy with sanctions.
Russia has responded with a ban on food imports from some Western countries.
Results released on Monday showed Alexander Zakharchenko, the self-declared prime minister in Donetsk, had won the poll to become the head of the region. His party also came first in the parliamentary election.
In Luhansk, current PM Igor Plotnitsky was declared the winner.
They are both expected to be officially sworn in later on Tuesday.
BBC
