Ukrainian forces have raised the national flag over the pro-Russian rebels' main stronghold of Slovyansk after flushing them out of the eastern industrial city, the country's defence minister has said.

"The Ukrainian national flag was raised over the Slovyansk city council before a lineup of Ukrainian soldiers," Ukraine's defence minister, Valery Heletey, said on Saturday in a statement posted on the president's official website.

The announcement came hours after the interior minister, Arsen Avakov, said a large number of pro-Russian separatists had fled the city after attacks from Ukrainian forces.

In a statement on his Facebook page, Avakov said: "A significant number of militants have left ... along the way, our battle group is 'greeting' them. They are suffering losses and surrendering."

The Interfax news agency quoted the leader of the Donetsk People's Republic, Aleksandr Borodai, as saying that his forces were pushed back by a "large-scale" offensive.

"Given the disproportionate numerical superiority of the enemy troops, units of the armed forces of the Donetsk People's Republic were forced to leave their previous positions on the northern sector of the front and go to pre-prepared [positions]," he said.

A rebel commander, who would only give his name as "Pinochet", told the AP news agency that the rebels had relocated to the nearby town of Kramatorsk.

Slovyansk was seized by rebels in April. It became the separatists' strongest redoubt in eastern Ukraine and a base for Igor Strelkov, a Russian-declared defence minister of the self-proclaimed Donetsk People's Republic.

Al Jazeera's Scott Heidler, reporting 80km from Slovyansk, said he had seen separatist tanks and armoured personnel carriers heading in the opposite direction to the town.

"Whether the rebels are able to regroup and then push back, that is left to be determined," he said. "But this appears to be the most significant movement in the past few weeks."