KIEV, Ukraine (AP) — Three pro-Russian militants died and 13 were wounded when Ukrainian troops repelled an attack on a National Guard base in the Black Sea port of Mariupol, Ukraine's interior ministry said Thursday.

A crowd of around 300 men armed with stun grenades and Molotov cocktails attacked the base, in the south-east part of the country late Wednesday, the interior ministry said in a statement. Servicemen inside fired warning shots but the attackers did not stop the assault and the army had to respond, it said.

There were no casualties among the Ukrainian servicemen, the ministry said, and 63 attackers were detained.

Speaking at the parliament Thursday morning, acting President Oleksandr Turchynov said the pro-Russian gang attacked the military base with automatic weapons.

The skirmishes come after the Ukrainian government announced an operation to retake control of Slovyansk and other cities in the restive east.

A well-armed, Russia-backed insurgency has sowed chaos in eastern Ukraine in the past weeks and on Wednesday insurgents seized armored vehicles and weapons from underequipped Ukrainian government forces. Militants have taken control of police stations and administrative buildings in at least 10 towns in the region, including Mariupol near the Russian border.

With tens of thousands of Russian troops deployed along the border with Ukraine, there are fears the Kremlin might use the instability in the predominantly Russian-speaking region as a pretext for seizing more territory beyond its annexation of Crimea last month.

The latest unrest came as diplomats from Ukraine, the U.S., the European Union and Russia prepared to meet Thursday for the first time over the burgeoning crisis that threatens to roil the new government in Kiev. With Ukraine struggling to contain the pro-Russian uprising, the Obama administration has said it is readying additional sanctions against Moscow and a boost in aid for the Ukrainian military in the coming days.