Six Ukrainian troops were killed by landmines in the restive separatist-held east Sunday, breaking a lull of several days in a conflict that began a year ago this week.
After weeks in which a shaky ceasefire deal appeared to be largely holding despite isolated clashes, the fatalities brought to nine the number of soldiers reported dead within just 48 hours.
And Ukraine President Petro Poroshenko reiterated a call at the weekend for international peacekeepers to be brought in to try to help end the conflict.
"Unfortunately, six Ukrainian soldiers have been killed today," army spokesman Oleksandr Motuzyanyk told AFP.
Four soldiers died when their vehicle came under missile fire while crossing a bridge near the government-held village of Schastya, not far from the separatist bastion of Lugansk in the northeast.
Ukrainian authorities said a rebel missile hit the bridge, which was mined, detonating a string of mines and causing the vehicle to explode.
"They were our mines. They're anti-tank mines our soldiers had placed there for protection," said Motuzyanyk.
Two others soldiers were killed when an anti-tank mine exploded a few kilometres outside the government-held southeastern port city of Mariupol, the spokesman added.
And on Saturday the government reported the deaths of another three soldiers, the first deaths announced in almost a week.
Mines have been laid across the country since the fighting broke out between pro-Russian separatists and the Ukrainian government in April last year.
More than 6,000 people have died since then.
Kiev and the West accuse Russian President Vladimir Putin of supporting the insurgency with troops, tanks and heavy weapons -- accusations he rejects.
Ukraine authorities agreed a ceasefire with pro-Russian separatists on February 12 in the Belarussian capital of Minsk.
The Organization for Security and Cooperation in Europe which is monitoring the deal on Friday the truce remained "somewhat fragile".
"As long as guns continue to be fired, and as long as substantial amounts of heavy weaponry continue to be concentrated in offensive and defensive positions, achieving a comprehensive and sustainable ceasefire will be difficult," said OSCE spokesman Michael Bociurkiw.
Skirmishes continue, notably around Donetsk and the port of Mariupol, one of Kiev's largest remaining strongholds in the east, he said.
He warned of a growing humanitarian crisis in the east due to the lack of hospitals, power, water and schools. "Children have nowhere to play or to learn," he said.
As divided Ukraine struggles to overcome the economic and humanitarian fallout, Poroshenko pledged this weekend to stick to the terms of the fragile ceasefire while saying his army had become increasingly capable of defending itself.
"Despite the fact that the Ukrainian armed forces, per my order, are strictly abiding by the ceasefire agreement, if there is a violation of it (ceasefire) we have the capability to respond," he said.
The comments came as Poroshenko inspected newly-acquired and revamped weaponry at the National Guard training centre, where he noted there had been a marked improvement in the defence manufacturing, now "100 percent" homemade.
In a separate statement he reiterated a call for an international peace force to help put pay to a year of trouble in the former Soviet state.
He said he expected this to be raised at upcoming talks between foreign ministers of France, Germany, Ukraine and Russia.
"Peacekeepers are not an alternative to the OSCE process," Poroshenko said, referring to the OSCE's 400 monitors enforcing the ceasefire. "We have to go this way," he told Channel Nine.