Ukrainian government forces reclaimed the port city of Mariupol from pro-Russian separatists in heavy fighting on Friday and said they had regained control of a long stretch of the border with Russia.

The advances are significant victories for the pro-European leadership in a military operation to crush the armed separatist rebellion that began in east Ukraine in April and hold the former Soviet republic of 45 million together.

¡°At 10:34 a.m. the Ukrainian flag was raised over City Hall in Mariupol,¡± Interior Minister Arsen Avakov wrote on Facebook, less than six hours after the attack began on the city of 500,000, Ukraine¡¯s biggest Azov Sea port.