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By Natalia Zinets
KIEV, July 5 (Reuters) - Ukrainian President Petro Poroshenko ordered the army to raise the national flag over the separatist stronghold of Slaviansk in eastern Ukraine on Saturday, after hearing reports government forces were in the city, the presidential web site said.
Slaviansk, seized by rebels in April, had become the separatists' strongest redoubt in eastern Ukraine and a base for Igor Strelkov, a Muscovite declared defence minister of the self-proclaimed Donetsk People's Republic.
Interior Minister Arsen Avakov said a large number of pro-Russian separatists had fled the city following sustained fire from Ukrainian forces.
Poroshenko's website said the head of the armed forces general staff had told him separatist fighters came under mortar fire as they tried to break through government forces' lines.
The separatists had lost one tank and other armoured vehicles, the statement said.
"The President gave the order ... for the state flag to be raised on Slaviansk's town council offices," the website said, referring to one of the rebels' main headquarters in the city.
In a statement on his Facebook page, Avakov said: "A significant number of militants have left Slaviansk ... Along the way, our battle group is 'greeting' them. They are suffering losses and surrendering."
Ukraine, which has lost more than 200 servicemen in fighting since the beginning of violence against the separatists, has concentrated much of its might around Slaviansk, a city of 130,000 people, during a so-called "anti-terrorist operation".
The government launched a fresh offensive against the separatists early on Tuesday after Poroshenko allowed a unilateral ceasefire to expire. (Writing by Thomas Grove; Editing by David Holmes)