Ukrainian troops repelled an overnight attack on their base in the southeast city of Mariupol, killing three of the assailants and wounding 13, Interior Minister Arsen Avakov said Thursday.
Around 300 people mounted the attack using guns and petrol bombs on the interior ministry troops, none of whom was hurt, Avakov said on his Facebook page.
"After attackers threw incendiary devices and Molotov cocktails at the military unit and opened fire at guard posts, the national guards fired warning shots," Avakov said.
After the attack continued, they turned their weapons directly on the assailants.
Sixty-three people were detained and security officials confiscated weapons, communication devices and "Russian cell phones", the statement said.
The operation was still ongoing with additional police patrolling the port city and special forces being helicoptered in as reinforcements, Avakov said.
The latest attack follows the seizure of administrative buildings by pro-Russian separatists in Mariupol and other towns across the restive east of the country.
High-level talks aimed at ending the escalating crisis involving the top diplomats of Russia, Ukraine, the United States and the European Union are due to start in Geneva on Thursday.
Kiev has struggled to reimpose its authority over its industrial heartland. A military operation to oust the separatists turned to fiasco Wednesday when pro-Moscow militants seized at least six armoured vehicles.