Ukrainian troops have repelled an overnight attack on their port base in the southeast city of Mariupol, killing three of the assailants and wounding 13 others, Arsen Avakov, the country's interior minister, has said.
About 300 people mounted the attack using guns and petrol bombs against interior ministry troops on Thursday, none of whom was hurt, Avakov said on his Facebook page, according to the AFP news agency.
"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.
The escalation comes as high-level talks involving the top diplomats of Russia, Ukraine, the United States and the European Union begin in Geneva on Thursday, aimed at resolving the crisis.
John Kerry, the US secretary of state, Sergey Lavrov, Russia's foreign minister, Andriy Deshchytsia, Ukraine's foreign minister and Catherine Ashton, the EU foreign chief, are meeting in a final attempt to engage with Russia before the west hits Moscow with additional economic sanctions.
Al Jazeera's Nick Spicer, reporting from Geneva, said the talks were a "clash of world opinion", adding that Russia is accusing the US of installing the interim administration in Ukraine while Washington accuses Moscow of fomenting trouble in eastern Ukraine.
The meeting comes a day after pro-Russian forces seized armoured vehicles and weapons from government troops, in a blow to the Ukrainian army's attempt to retake control of Solvyansk and other cities in the east.
Residents in the eastern city of Kramatorsk, 15km south of Slovyansk, encircled a column of Ukrainian armoured vehicles. Masked gunmen, wearing combat gear and identifying themselves as pro-Russian militia, then forced the Ukrainian soldiers to surrender.
One soldier said the Ukrainian troops defected to the pro-Russian side, but another suggested that they were forced at gunpoint to hand over the vehicles.
"How was I supposed to behave if I had guns pointed at me?" the soldier, who did not identify himself, asked a resident.
Pro-Russia forces also seized the police headquarters and the administration building in Slovyansk, demanding broader autonomy for eastern Ukraine and closer ties with Russia.
Kiev continues to blame Moscow for the unrest in the region, as similar seizures occurred in at least 10 other cities in the east.
Vladimir Putin, the Russian president, continues to deny any involvement, and on Tuesday said that "the sharp escalation of the conflict places the country in effect on the verge of a civil war".
Anders Fogh Rasmussen, the NATO secretary general, said the alliance would also respond to what he called Russian aggression in Ukraine.
|US warns Russia of further sanctions
"We will have more planes in the air, more ships on the water and more readiness on the land," he said.
The measures however amount to more air and sea patrols in the Baltic, some distance from the trouble in Ukraine.
The US president, Barack Obama, also gave warning on Wednesday that Russia could expect further sanctions if it steps up support for separatists in eastern Ukraine.
"Mr Putin's decisions are not just bad for Ukraine, over the long term they're going to be bad for Russia," he said.