Ukraine's President Petro Poroshenko has ordered army reinforcements to key southern and eastern cities in case of a new rebel offensive.
Mr Poroshenko said the units were to protect Mariupol, Berdyansk, Kharkiv and the north of Luhansk region.
He spoke after meeting security chiefs in the wake of disputed polls in rebel-held areas of Donetsk and Luhansk.
He said he was still committed to the current peace process but has proposed cancelling a key plank of the plan.
The peace process was laid out in a 5 September ceasefire deal agreed in Minsk, Belarus.
Angered by the elections, Mr Poroshenko has proposed scrapping a law that gives special status to Donetsk and Luhansk.
Both regions staged swearing-in ceremonies for their pro-Russian leaders on Tuesday, following the elections there on Sunday.
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President Petro Poroshenko: "The election farce of 2 November jeopardises the peace process"
Alexander Zakharchenko was inaugurated president of the Donetsk People's Republic while Igor Plotnitsky was sworn in as president of the Luhansk People's Republic.
The polls were held against the background of a conflict that has killed more than 4,000 people 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.
Both the government and rebel sides have repeatedly violated the ceasefire.
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Analysis: BBC's David Stern in Kiev
Fragile and wobbly as it may be, the ceasefire between Ukrainian government troops and Russian-backed militants has not completely broken down. And that's the most important thing at the moment.
The Minsk peace plan was Mr Poroshenko's attempt to buy himself some breathing space, and it still seems he is wedded to it. But the domestic pressure on President Poroshenko to take decisive action against the creation of a de facto independent rebel "statelet" is growing.
And fears are growing that the insurgents, with Russian assistance, may themselves soon launch a major offensive. Sending more troops to eastern Ukraine may be a desperate show of strength - an attempt to appear to be doing something amid a worsening situation. Or it could be a credible reaction to a real, Russian-backed threat.
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'Serious danger'
Ukraine and its Western allies have condemned the elections in the east.
EU foreign policy chief Federica Mogherini said on Tuesday that the polls were "illegitimate and risk putting in serious danger the path of dialogue and peace".
Alexander Zakharchenko is inaugurated president of the Donetsk People's Republic, 4 Nov
Alexander Zakharchenko is inaugurated president of the Donetsk People's Republic
She added: "The main risk I see now is that we close this window of opportunity for internal dialogue and for dialogue with Russia."
New Nato chief Jens Stoltenberg said at the same press briefing that Russian troops "were moving closer to the border with Ukraine".
A Nato spokesman also said the reported movement of soldiers and trucks within separatist areas of eastern Ukraine was "alarming and dangerous".
'Pessimistic scenario'
At the meeting of his security chiefs, Mr Poroshenko said the Ukrainian reinforcements would be for the "construction of fortifications" against a "possible offensive in the direction of Mariupol, Berdyansk, Kharkiv and Luhansk north".
He said Ukraine remained "a supporter of the peace plan" and would adhere to its terms, which were agreed in Minsk by delegations from Ukraine, Russia, rebels and the Organization for Security and Co-operation in Europe.
Ukrainian servicemen near the eastern town of Debaltseve, 4 Nov
Both the government and rebel sides have repeatedly violated the ceasefire
However, he said: "This does not mean that Ukraine will not be ready for a decisive action in case of a pessimistic and negative scenario."
Addressing the nation on TV late on Monday, President Poroshenko described Sunday's "pseudo-elections" in Donetsk and Luhansk as a "farce at gunpoint" which would never be recognised as legitimate.
The polls, he said, were "a gross violation" of the Minsk agreement, which required the eastern regions to hold local elections under Ukrainian law in December.
Mr Zakharchenko said on Tuesday he was ready for peace talks "with anyone, including Petro Poroshenko".
But he said: "Ukraine has to understand that the [Donetsk People's Republic] is already another state."
Mr Zakharchenko also laid claim to the entire Donetsk region, parts of which remain under Ukrainian government control.
The separatist insurrection erupted in the east after Russia annexed Ukraine's southern Crimea peninsula, weeks after Ukraine's pro-Russian President, Viktor Yanukovych, was forced out of office by mass protests in Kiev.
source: bbc.com
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