* Attack bloodiest in 10 days of unrest in eastern Ukraine
* Took place on eve of Geneva talks on Ukraine crisis (Changes dateline, adds interior ministry figures for casualties)
MARIUPOL, Ukraine, April 17 (Reuters) - Separatists attacked a base of the Ukrainian national guard in an eastern city overnight and, in shooting that ensued, three of them were killed, the interior minister said on Thursday.
The minister, Arsen Avakov, said in a post on Facebook that an armed group of about 300 attacked the base in Mariupol on the Sea of Azov with guns and petrol bombs. National guard members first fired warning shots but then opened fire as the attack continued, Avakov said
"According to preliminary data, three attackers were killed, 13 wounded and 63 detained," Avakov said.
That would make the attack, which occurred on the eve of international four-way talks in Geneva on the crisis in Ukraine , the deadliest since separatist rebellions broke out in Russian-speaking eastern areas of Ukraine over 10 days ago.
No guardsmen were killed, Avakov said, but a major security sweep was underway in the area to detain other armed members of the group.
"Given the aggressive nature of the attack on the base, an interior ministry group has been strengthened by Omega special forces. Helicopters have been sent in," Avakov said.
Arms had been seized from the attackers as well as communications equipment, he said.
The Mariupol city administration was seized at the weekend by separatists. It was not known if it was still under their control.
Earlier, an interior ministry statement said armed protesters had arrived at the base on Wednesday night and demanded that the national guard troops back their revolt against the country's new pro-Western government.
Pro-Russian separatists have taken over buildings in at least 10 cities in eastern Ukraine, attacks which the Ukrainian government has said are orchestrated by Russian agents.
Russia backed the now-ousted president Viktor Yanukovich through three months of protests against his rule. Since he fled Ukraine for Russia in late February, Moscow has refused to recognise the legitimacy of Ukraine's pro-Europe rulers. (Additional reporting by Aleksander Vasovic in Mariupol; Writing by Richard Balmforth; Editing by Larry King)