KIEV, Ukraine (CNN) -

Ukrainian security forces cleared pro-Russian separatists out of two eastern cities Saturday -- significant developments, though hardly conclusive, in the effort to bring unity and stability to the European nation.

The Ukrainian flag was raised Saturday over Slovyansk and Kramatorsk, a pair of flashpoint cities that have been strongholds of groups advocating independence from the Kiev-based national government for months.

President Petro Poroshenko's officed announced Saturday that separatists had left Slovyansk.

Clearing this key city was very difficult, since militants hid inside residential buildings and other facilities where civilians are, according to Andriy Lysenko, a spokesman for Ukraine's National Defense and Security Council.

Interior Minister Arsen Avakov said most of the militants had left.

"They are running. This morning intelligence suggests that (separatist militant leader Igor) Girkin and a significant number of fighters have fled Slovyansk. ... Some remain. Rest going to Gorlovka," he wrote.

Pavel Gubarev, a leader of the militants, confirmed that Igor Girkin has fled Slovyansk with many other leaders toward the main city of Donetsk. The separatists vowed to continue their fight.

Later Saturday, Prime Minister Arseniy Yatsenyuk's office announced that the eastern city of Kramatorsk was also free of separatists.

Videos posted to social media showed the Ukrainian flag being raised about Kramatorsk.

These reports come days after Ukrainian forces began military operations in the east of the country, marking a definite end to a unilateral cease-fire in place for 10 days.

The speaker of Ukraine's Parliament, Oleksandr Turchynov, told lawmakers the government's "anti-terror operation" against pro-Russia separatists had been "renewed."

Ukrainian armed forces have been attacking "terrorists' bases," authorities said.

Poroshenko had said his country would not renew a cease-fire with the separatists, vowing instead to "attack and liberate our land."

Russia and Ukraine have been at odds since late last year, when former Ukrainian President Viktor Yanukovych refused to sign a cooperation agreement with the European Union, setting off nationwide protests.

The deadly protests led to Yanukovych's ouster, the loss of Crimea to Russia and a pro-Russia separatist rebellion.