President Petro Poroshenko has ordered more troops to key southern and eastern Ukraine cities in case of a new rebel offensive.
Petro 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.
Petro Poroshenko 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 September 5 ceasefire deal agreed in Minsk, Belarus.
Angered by the elections, Petro 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 November 2.
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.
Ukraine and its Western allies have condemned the elections in the east.
At the meeting of his security chiefs, Petro 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 (OSCE).
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