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By Richard Balmforth and Pavel Polityuk KIEV (Reuters) - Ukraine's military said on Monday its forces were battling a Russian tank battalion for control of a vital airport in the east of the country as President Petro Poroshenko accused Moscow of "direct and open aggression" against his country. Ukrainian coastguards meanwhile searched for two seamen missing after one of their patrol boats was sunk in the Sea of Azov by artillery fire from pro-Russian separatists on the shore. Several hundred Ukrainian forces are bogged down near Ilovaysk, east of the region's main city of Donetsk, and have been trying to break out of encirclement by Russian-backed separatists for several days. Poroshenko, speaking at a military academy in Kiev, said Russia's direct involvement in the war against the separatists in eastern Ukraine had tipped the balance on the battlefield and was the main reason for recent reversals.