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Evgeniy Maloletka, Associated Press
Ukrainian military vehicles drive towards Debaltseve on the outskirts of Artemivsk, eastern Ukraine, Sunday, Feb. 8, 2015. The government-held town of Debaltseve, a key railway junction, has been the epicenter of recent battles between Russian-backed separatists and Ukrainian government troops. For two weeks, the town has been pounded by intense shelling that knocked out power, heat and running water in the dead of winter.

DONETSK, Ukraine — A powerful explosion rocked a chemical plant and set it on fire Monday outside the separatist stronghold of Donetsk in eastern Ukraine. Rebels said the plant was hit by government shelling.

No casualties or damage were immediately reported from the blast, which could be heard in downtown Donetsk. The city lies in the middle of Ukraine's industrial heartland and explosions like this have long been feared.

An armed conflict between Russia-backed separatists and Ukraine government forces has killed more than 5,300 people and displaced more than a million people in eastern Ukraine.

The rebel mouthpiece Donetsk News Agency said Monday the chemical plant to the west of the city was hit by a Ukrainian artillery shell but the fire was put out and it posed no danger.

Heavy shelling around Donetsk overnight killed two people in the city, rebel official Eduard Basurin said. In neighboring areas under government control, at least seven people were killed Sunday, the Donetsk regional police chief said.

Ukrainian military spokesman Andriy Lysenko, meanwhile, said Monday that about 1,500 Russian troops had crossed the border into Ukraine via rebel-controlled border posts over the weekend. He did not provide any proof.

Kiev has previously reported the presence of Russian military troops in eastern Ukraine. Russia has denied that it is supplying the rebels with either troops or heavy weapons, but Western military experts say the sheer amount of new heavy weapons in eastern Ukraine belies the Russian denial.

The leaders of Germany, France, Russia and Ukraine aim to hold a summit on Wednesday to breathe new life into a much-violated September peace plan for eastern Ukraine. Preparations for the summit were taking place Monday in Berlin, while rebel leaders, representatives of Russia, Ukraine and the Organization for Security and Cooperation in Europe are expected to meet on Tuesday.

In Moscow, Russian radio station Govorit Moskva quoted President Vladimir Putin's spokesman, Dmitry Peskov, as dismissing speculation that German Chancellor Angela Merkel had issued the Russian leader an ultimatum at talks Friday in Moscow.

"Nobody has ever talked with the president with ultimatums, and they could not even if they wanted to," Peskov was quoted Monday as saying.

In Brussels, French Foreign Minister Laurent Fabius said the European Union will hold off for now on expanding sanctions against Russia and the separatists for their actions in Ukraine and await developments from the peace talks. He said the need for more sanctions will be re-assessed Monday.

EU foreign ministers have backed fresh diplomatic efforts to end the conflict in Ukraine, rejecting calls by some U.S. politicians to provide lethal defensive weapons to Kiev.