Ukraine said Friday that renewed fighting with Russian-backed separatists is undermining a fragile ceasefire in the east of the country, and urged Western backers to intervene.

The halt to fighting agreed last July raised hopes for a resolution to the bitter trench war that was sparked in 2014 by the Kremlin's annexation of Crimea.

But after the longest truce since the beginning of the war, eight Ukrainian servicemen have been killed in clashes with separatist fighters since mid-February, and on Friday separatists announced that three of their own had been felled.

"We have seen an escalation in the conflict in recent weeks," the head of the Ukrainian president's office, Andriy Yermak, said Friday.

Yermak, who was speaking via video link at a conference held by the Brussels-based European Policy Centre think tank, blamed the uptick in violence on Moscow.

He urged Western peace brokers including France, Germany and the United States "to step up their efforts" to settle the conflict.

Observers say the latest escalation might be a Kremlin response to Ukrainian sanctions imposed by President Volodymyr Zelensky against a powerful pro-Russian lawmaker and Vladimir Putin's close ally last month.

"Russia is putting pressure on Kiev, including threatening to expand the scale of aggression," Oleksandr Lytvynenko, director of the National Institute for Strategic Studies in Kiev, told AFP.

Ukraine and its Western allies accuse Russia of sending troops and arms to support the separatists, claims Moscow denies.

Kiev has blamed the recent clashes on Moscow and separatist leaders, and accused them of using banned military hardware.

Heavy weapons such as tanks and large-calibre mortars are banned under the so-called Minsk II agreement that was signed in February 2015 to settle the conflict.

Humanitarian groups which provide aid to war-scarred Donetsk and Lugansk regions have confirmed that situation in the conflict zone has deteriorated.

"We had to postpone some of our trips because of the shelling in the evenings," Florence Gillette, head of the Red Cross delegation in Ukraine, told AFP in February, adding that this was a first for months.

Ukrainian soldiers on the frontline with Russia-backed separatists in the Donetsk region last month
Ukrainian soldiers on the frontline with Russia-backed separatists in the Donetsk region last month AFP / Anatolii STEPANOV

The Kremlin for its part earlier this week blamed the escalation on Ukrainian forces and said it was "deeply concerned by growing tensions" on the frontline.

"We are recording more and more shelling from the Ukrainian side," Kremlin spokesman Dmitry Peskov said, calling on Kiev to adhere to agreements to end the war.

He urged "everyone who can" to use their influence to avoid renewed full-fledged war.

European Council chief Charles Michel said during a visit to Ukraine this week that the EU has no plans to lift economic sanctions against Russia for its role in the conflict.

He said that Russia has not reciprocated "Ukraine's positive steps" towards ending the war, and described Moscow as "a party to this conflict, and not a mediator".

The fierce fighting has claimed more than 13,000 lives since 2014, but the number of new fatalities has been decreasing in recent years.

Still, tensions between the two neighbouring countries remain high.

Ukraine last month imposed sanctions against Viktor Medvedchuk, a 66-year-old close ally of Putin, accusing him of "financing of terrorism" due to his business in Russia.

For Steven Pifer, a former US ambassador to Ukraine and analyst with the Brookings Institution, the move could have precipitated the uptick in recent violence.

"It now appears the Kremlin is ratcheting things up. This could be due to Moscow's unhappiness over the sanctions on Medvedchuk," Pifer told AFP.

However he downplayed warnings that the sides could resume full-blown war.

"What would Moscow gain? It could perhaps grab some more Ukrainian territory, at the risk of Russian casualties and harsher Western sanctions," he said.