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

Rebels in the city of Donetsk reported 15 dead between civilians and 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 centered. Ukrainian presidential adviser Yury Biryukov cited the General Staff as saying three Ukrainian soldiers died in combat and that another 30 were wounded.

Each side is blaming the other for sparking this round of unrest.

Ukraine’s 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 cease-fire agreement.

The head of the separatist armed forces, Vladimir Kononov, said his fighters had engaged only in defense measures after an all-out assault by the Ukrainian army.

“At around 03:45 a.m., 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 450-kilometer (280-mile) front.

Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of inciting the fighting. Ukraine and the West assert 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 four troops with Russian military intelligence agency GRU were killed in Marinka. Lubkivskyi did not explain how that information was acquired.

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