Ukrainian troops backed by tanks and fighter bombers suffered their first losses on Wednesday as they pressed on with a renewed offensive against pro-Kremlin insurgents that has drawn Russian ire but also vital US and German support.

The return of all-out fighting in Europe's worst security crisis in nearly two decades set off a new international scramble to dampen hostilities in the strategic ex-Soviet state.

"We will not ease up... in looking for diplomatic solutions to the conflict. But we are still a long way from where we would like to be," German Chancellor Angela Merkel at a joint press appearance with NATO head Anders Fogh Rasmussen.

Ukrainian President Petro Poroshenko on Monday took the risky step of contradicting his European allies for the first time since his May 25 elections by ripping up a 10-day truce.

But Merkel appear to absolve Poroshenko for the truce's failure and accused the rebels of rejecting Kiev's peace push.

"It is unfortunate that during the 10-day unilateral... ceasefire proclaimed by the Ukrainian president there was no significant reaction to the peace plan, and that the separatists have so far not accepted such a ceasefire," Merkel said.

Russian and Ukrainian foreign ministers were due to meet their counterparts from Germany and France in Berlin later on Wednesday in a bid to find some common ground that could establish a long-term truce.

But Ukrainian leaders said their military operation was progressing with a resolve to stand up to what they see as a last-ditch Russian effort to halt their new westward drive.

"Everything is going according to plan. The advantage is on our sides," acting Defence Minister Mykhailo Koval told reporters.

A spokesman for the national security and defence council in Kiev said militias killed four Ukrainian soldier and wounded 10 others in separate mortar fire attacks along the Russian border.

They were the first casualties reported by Kiev in the second stage of the low-scale war. Regional officials on Tuesday had also confirmed the deaths of four civilians in a roadside attack on their bus.

A rebel spokesman in the industrial province of Lugansk told Russian media that government forces' shelling had killed 10 civilians overnight.

An AFP team in the region could not immediately verify the claim.

But the Russian foreign ministry once again called on the new "Ukrainian leaders, if they are still able to sanely assess the consequences of their criminal policies, to stop attacking civilians and shelling their own villages."

- US blames militants -

Poroshenko decision was immediately followed with the military's announcement of a "massive" operation in the eastern rustbelt designed to pound the rebels into submission and end the country's worst crisis since independence through force.

Russian President Vladimir Putin said Poroshenko now bore full responsibility for the deaths that would follow in an offensive that has already claimed more than 450 lives in 11 weeks.

The warning appeared designed to show that Putin had until now been willing to work with the new Ukrainian leader despite the February ouster of a pro-Kremlin administration in what Moscow branded a "coup".

Putin and Poroshenko had spoken by phone regularly before the ceasefire in a joint bid to draft a peace plan that could ultimately heal the ex-Soviet neighbours' severed ties.

But tensions between Moscow and Kiev now appear ready to soar again -- a worrying turn of events for European leaders who will face added pressure to unleash punishing sanctions against Russia that would likely hurt their own economies.

- Strong US support -

But Washington immediately rallied to Kiev's defence with a vigour reflecting the Cold War-like chill that has recently enveloped its relations with the Kremlin.

"President Poroshenko put in place a seven-day ceasefire. He abided by it. He extended it for three days, but the fact remained that the separatists, many of them were not adhering to it, and he has a right to defend his country," said US State Department spokeswoman Marie Harf.

"The Ukrainian forces have a responsibility to defend their territory and their people. And what they are seeing is aggression by Russian-backed separatists that they have an obligation to respond to."

Washington has strnogly criticised Putin for failing to call the rebels to lay down their weapons and relinquish control of roadblocks and border crossings across Lugansk and Donetsk.

burs-zak/ec