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 the latest unrest.

Ukraine’s General Staff said 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.

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 U.S. State Department said it was disturbed by the unrest and said any rebel attempts to seize Ukrainian territory would have costs for Russia, which it accuses of guiding the separatists.

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