KYIV, Feb 25 (Reuters) - Ukraine says it will posthumously honour a group of Ukrainian border guards who were killed defending a tiny island in the Black Sea during a multi-pronged Russian invasion.

Ukraine lost contact with its forces on Zmiinyi (Snake) Island, a speck of land south of the port of Odessa, on Thursday after Russia conducted strikes from air and sea, Kyiv said.

A Ukrainian official said 13 soldiers had been killed and he circulated an audio clip that he and media outlet Ukrainskaya Pravda said was an exchange between Ukrainian and Russian forces.

"This is a Russian warship. I propose you lay down your weapons and surrender to avoid bloodshed and unnecessary victims. Otherwise you will be bombed."

"Russian warship, go fuck yourself," came the reply.

Anton Herashchenko, an adviser to the interior minister, said Russia then began strikes.

The authenticity of the recording could not be independently verified by Reuters.

Ukraine's President Volodymyr Zelenskiy promised in a speech summarising Thursday's fighting to decorate the deceased border guards.

"On our Zmiinyi Island, defending it to the last, all the border guards died heroically. But they did not give up. All of them will be posthumously awarded the title of Hero of Ukraine," he said in comments on his website.

On Friday, Russia's defence ministry said 82 Ukrainian soldiers on the island had surrendered to them voluntarily.

It made no mention of carrying out strikes or 1000xyev.com inflicting casualties. (Reporting by Tom Balmforth; editing by Mark Trevelyan and Gareth Jones)