A major battle has erupted on the western edge of the main separatist rebel stronghold in eastern Ukraine, leaving more than a dozen people dead and threatening to tip the country back into full-blown war.

Rebels in the city of Donetsk reported 15 dead on Wednesday, some civilians and some combatants, in territory under their control.

The Ukrainian interior ministry said at least one person was killed in Marinka, the government-held town where fighting was centred. Ukrainian presidential adviser Yury Biryukov cited the Ukrainian general staff as saying two Ukrainian soldiers died in combat and that another 30 were wounded.

Each side is blaming the other for sparking this round of unrest. The general staff said in a statement that rebels deployed around 1,000 fighters and dozens of tanks and self-propelled artillery systems in a major offensive that started before dawn.

The statement said the attack was a flagrant violation of a February ceasefire agreement.

The head of the separatist armed forces, Vladimir Kononov, said his fighters had engaged only in defence measures after an assault by the Ukrainian army. “At around 3:45am the Ukrainian side carried out a provocation by shelling our positions practically along our entire front,” Kononov said.

Kononov listed several positions far apart from one another along the 280-mile (450km) front.

The Ukrainian prime minister, Arseniy Yatsenyuk, accused Russia of inciting the fighting. Ukraine and the west assert that Moscow supplies rebels with manpower and powerful weapons. Russia rejects those claims as unfounded.

Yatsenyuk urged the leaders’ meeting at the G7 summit in Germany this weekend to condemn Russia. “The international community must come up with a correct and appropriate response to Russian aggression,” Yatsenyuk said.

Ukrainian security services spokesman Markian Lubkivskyi said his agency had information that four troops with Russian military intelligence agency GRU had been killed in Marinka. Lubkivskyi did not explain how that information was acquired.

Implementation of the ceasefire sealed in Belarus in February has foundered amid a proliferation of low-intensity battles that have now been overshadowed by developments in Marinka.

Negotiators from the warring sides met for new talks in Belarus on Tuesday, but the session concluded without obvious progress. Another meeting is planned for the weekend.

Russia has said it is unnerved by the violence, which it is blaming on Ukraine. “We here in Moscow are closely watching it and feel extreme concern about the provocative actions by the Ukrainian armed forces,” said Kremlin spokesman Dmitry Peskov.

Regular reports of casualties among government and separatist fighters have continued unabated since February, but deaths among noncombatants had almost ceased. That trend has been disrupted in recent days in an indication that the warring sides are again increasingly resorting to indiscriminate shelling.

The head of the government-controlled part of the Luhansk region, Hennadiy Moskal, said on his website that an elderly couple died when their car was struck by a mortar on Wednesday morning, 20km inside government-held territory.

Russia’s Interfax news agency cited a rebel representative as saying that one resident in the town of Yenakiieve was killed near a waste treatment plant during a 20-minute barrage. Ukraine’s military denies it attacked Yenakiieve.

An array of social media postings showed the aftermath of shelling on Wednesday in locations deep within the rebel citadel of Donetsk. The war has killed more than 6,400 people since April 2014.

The armistice signed in February requires both sides to pull back heavy weapons from the front line, but international observers vetting that process regularly note violations across the board.

The Organisation for Security and Cooperation in Europe said in its most recent status report that its monitors heard 126 explosions on Tuesday from their observation point inside Donetsk. The report did not specify if the blasts indicated incoming or outgoing fire.