President Petro Poroshenko said "about 200 insurgents" had staged a pre-dawn raid on Novolaspa -- a village halfway between the separatists' de facto capital Donetsk and the Kiev-held southern port of Mariupol -- that caught government soldiers off-guard.
Chief of Staff General Viktor Muzhenko "informed the president that the Ukrainian forces gave a fitting rebuff and repelled all the attacks," the presidency said.
But the defence ministry later reported the militants mounting a second attack on the same village whose outcome was not immediately clear.
Local pro-Kiev officials told AFP that separatist fighters had also launched several waves of Grad missile attacks on the eastern outskirts of Mariupol itself.
The Ukrainian foreign ministry called the clashes "a dangerous indication of a further escalation to come".
But the rebels said Kiev's claims made no sense because Novolaspa had always been one of their frontline outposts.
"The armed forces of Ukraine simply put the village under a heavy shelling attack," a local separatist official told the rebels' main news site.
Kiev on Monday reported the death of one soldier while the insurgents accused government forces of killing three civilians in the rebel-held bastion of Horlivka.
The two self-proclaimed republics of Donetsk and Luhansk launched their revolt shortly after the February 2014 ouster of a Moscow-backed president in Kiev and Russia's subsequent seizure of Ukraine's Crimea peninsula.
The clashes have killed more than 6,800 people and sent Moscow's relations with the West crashing to their lowest point since the Cold War.
The crisis has also left 1.4 million homeless and sent Ukraine's economy -- heavily dependent on exports from the industrial east of the country -- into a tailspin.
The West is still pinning hope on a February truce agreement that has often been ignored but also kept fighting limited to flashpoints in the Russian-speaking east of the former Soviet state.
The latest reported clashes came a day after a special monitoring mission from the Organization for Security and Co-operation in Europe (OSCE) saw several of its armoured vehicles torched outside its headquarters in Donetsk.
The European Union on Monday called the incident "unacceptable".
- 'Ready for anything' -
Poroshenko said Monday's reported tank battles around Novolaspa and Sunday's attack on the OSCE vehicles were all part of insurgent attempts to raise tensions and erase any achievements of the February truce.
But the militants accused Kiev of trying to gain back territory lost in fighting and now under pro-Russian control.
"We are on constant alert for a possible new wave of military activities," rebel chief Alexander Zakharchenko warned on Friday.
"We are ready for anything."
Tensions are rising ahead of the two separatist regions' planned elections this autumn, which Kiev has already condemned as illegal.
A recent rise in the number of protests against the OSCE mission has also alarmed European leaders who had hoped to see the final terms of a peace agreement concluded by the end of the year.
Germany Chancellor Angela Merkel and French President Francois Hollande have held two teleconferences with Poroshenko and Russian President Vladimir Putin in the past month, aimed at reviving the peace process and tackling what have become increasingly intractable disputes.
Poroshenko called on Monday for "urgent" new consultations between the four sides' foreign ministers about the purported rebel assault.
But initial indications suggested the talks were making limited progress.
Moscow said Russian Foreign Minister Sergei Lavrov told his Ukrainian counterpart Pavlo Klimkin that Poroshenko needed to finally engage the rebels in direct negotiations.
The Ukrainian leader has vowed repeatedly never to hold face-to-face talks with "terrorists".
Kiev said Klimkin countered that Lavrov should encourage Putin "to think things over" and drop his alleged support for the revolt.