Several pro-Russian militants have been killed in clashes with Ukrainian security forces at a National Guard base in southeastern Ukraine.
Ukraine's interior ministry said a crowd of 300 men attacked a base in the Black Sea port of Mariupol.
The separatist insurgents launched stun grenades and Molotov cocktails in an attempt to raid the base.
Kiev said servicemen inside opened fire when the attackers failed to stop after warning shots were fired. Three insurgents were killed and 63 detained.
"Through joint efforts by the armed police and national guard the attacking gang was dispersed after a short battle, most of them were cornered and disarmed," Interior Minister Arsen Avakov said.
There were no casualties among the Ukrainian servicemen, the ministry said.
Briefing the parliament on the incident, Ukraine's acting President Oleksandr Turchynov said the pro-Russian forces attacked the military base with automatic weapons.
The incident came amid rising tensions in the region, as Kiev launched a military operation to regain control of buildings seized by separatist insurgents demanding autonomy and closer ties with Russia.
Kiev has alleged that Russia's secret services helped and, in some cases, directed the attacks to fuel chaos and legitimise a possible military operation there.
Russian president Putin has rejected claims of Russian forces' presence in eastern Ukraine as "nonsense".
After the seizure of Crimea from Ukraine in March, Russia has amassed tens of thousands of troops along its western borders.
Meanwhile the US, Russia, the EU and Ukraine have started diplomatic talks to resolve the crisis in Geneva.
The US and the EU are demanding Russia pulls back troops from the border and put an end to the occupations in East.