Ukraine braced for escalation as rebels elect Donetsk chief
President Petro Poroshenko has said he will deploy additional troops to the east where fighting continues to rage between government forces and pro-Russian separatist fighters.
Mr Poroshenko said the units would defend cities still under government control against possible incursions.
His remarks came after Ukrainian security officials claimed over the weekend that Russia has intensified the transfer of troops and military equipment to separatist rebels.
Moscow denies it aids the rebels.
"Several new units have been formed that will enable us to repel possible attacks in the direction of Mariupol and Berdyansk, Kharkiv, and areas north of Luhansk and the Dnipropetrovsk region," Mr Poroshenko said, according to a statement on the presidential website.
Tensions have spiked since the weekend, when rebels held an election that was condemned by the West and Ukraine as illegal and destabilising.
"In recent days, Russia-supported separatists have publicly stated their intention to expand the territory under their control. We strongly condemn ongoing separatist attacks in Mariupol and Dubalsiva and around the Donetsk airport," US state department spokeswoman Jen Psaki told reporters in Washington.
"Any attempt to push further into Ukraine would be another violation of Ukraine's sovereignty and territorial integrity and a gross violation of the Minsk agreements signed by Russia, Ukraine and the separatists," she added, referring to the much-violated ceasefire agreed upon in September.
Ukraine and Western governments say Sunday's poll gravely endangered the ceasefire that envisioned local elections across the whole of the east but under Ukrainian law. Russia, however, quickly lent its support to the vote.
On Tuesday morning, the separatist leader in the Donetsk region, Alexander Zakharchenko (38) was sworn in as head of a self-declared secessionist territory.
He was inaugurated in a heavily guarded theatre in the city of Donetsk, the main stronghold of a rebel-controlled territory.