Ukraine and pro-Russian separatists accused each other on Wednesday of attempting to break a six-month-old peace agreement, with a rebel official warning of the threat of "a big war" if the ceasefire crumbles.
Ukraine's allies expressed particular concern this week after Kiev's military reported the heaviest rebel artillery attacks in six months and fresh fighting near the strategic port city of Mariupol.
Rebels have, in turn, accused government troops of increased shelling in defiance of the peace deal, brokered in the Belarus capital Minsk in mid-February to end a conflict in which over 6,500 have been killed.
Russian President Vladimir Putin discussed the recent escalation in fighting in east Ukraine with his Security Council on Wednesday, a Kremlin statement said. It blamed Kiev for shelling civilians as well as OSCE monitors.
"We unfortunately can see the limitation of the implementation of the Minsk agreements on the part of Kiev, unfortunately we can see provocations on the part of Kiev," rebel envoy Denis Pushilin said in a briefing.
"Kiev most likely also understands that if the Minsk process will be halted as it is ... this means war ... not only in the Donbass (eastern Ukraine), but this may be a big war," he said.
Ivica Dacic, the chairman of security watchdog the Organization for Security and Cooperation in Europe, on Tuesday described the latest escalation in separatist eastern regions as "alarming" and called on both sides to adhere to the ceasefire.
The OSCE has reported a significant increase of ceasefire violations in areas east and north of Mariupol.
"All these cases are examples of how the Russian side, along with Donetsk and Luhansk, is trying to derail the implementation of the Minsk agreements," Ukrainian Foreign Minister Pavlo Klimkin said at a briefing in Kiev.
Government troops are boosting their defenses in the conflict zone with new weapons and equipment, said Olexander Turchynov, secretary of Ukraine's National Security and Defense Council, adding the move was in line with the peace deal.
The conflict broke out in April last year, when separatists rebelled in mainly Russian-speaking eastern Ukraine against the rule of Kiev's new Western-looking government.
An escalation in the eastern conflict poses one of the greatest risks to Ukraine's bid to shore up its war-torn economy, the International Monetary Fund said last week.
Ukraine was due to meet with creditors later on Wednesday for make-or-break debt restructuring talks aimed at plugging the country's $15 billion funding gap.