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Evgeniy Maloletka, Associated Press
Ukrainian servicemen from battalion "Kiev1" check passing cars near Marinka, Donetsk region, eastern Ukraine, Monday, June 4, 2015. 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. The Ukrainian Interior Ministry said at least one person was killed in Marinka, the government-held town where fighting was centered.

KIEV, Ukraine — Ukrainian President Petro Poroshenko on Thursday warned of a possible large-scale offensive by separatist forces in the east, one day after a major battle erupted on the western edge of the main separatist rebel stronghold.

Heavy fighting on the outskirts of Donetsk on Wednesday was the most severe in months. A cease-fire called in February has been violated almost daily, but hostilities had tapered off after the agreement.

The fighting was heaviest in in the village of Marinka. Ukrainian officials said five servicemen were killed on Wednesday, four of them in or around Marinka. Late Wednesday, the minister of defense for the Donetsk rebels, Eduard Basurin, said 14 rebels and five civilians had been killed by Ukrainian fire during the day, but did not specify locations.

Ukrainian military spokesman Col. Andriy Lysenko claimed Thursday that about 80 rebels were killed in the fight, along with two civilians.

There were no immediate reports of fighting on Thursday, but the intensity of the combat the day before raised fears of full-scale fighting resuming in the war that has already killed more than 6,400 people since April 2014.

In an address to the Ukrainian parliament, Poroshenko spoke of a "colossal threat of the resumption of large-scale hostilities by Russian and terrorist forces" and said there are now 9,000 Russian troops in the rebel-controlled area.

The monitoring mission of the Organization for Security and Cooperation Europe said its observers saw at least a dozen tanks moving from the rebel-controlled city toward the western outskirts in the run-up to the battle.

Mission spokesman Michael Bociurkiw on Thursday urged all sides in the conflict to "exercise maximum restraint." He also told reporters that observers in recent days had found that some heavy weapons which had been pulled back by both sides were missing from the areas where they were being stored.

"This suggests non-compliance" with the cease-fire accord, which called for both sides to pull back large weapons in order to create a buffer zone, Bociurkiw said.

An Associated Press reporter on Thursday saw two Grad missile launchers driving toward the government-controlled town of Artemivsk; under the cease-fire accord, such weapons were to be withdrawn from that area.

Russia has strongly denied supplying the rebels either with weapons or with recruits, despite mounting evidence suggesting otherwise.

Nataliya Vasilyeva and Jim Heintz in Moscow and Yevgeny Maloletka in Donetsk contributed to this report.