Ukrainian border guards, who were caught by Russian soldiers while defending a small island in the Black Sea swore at the soldiers rather than surrender.

The 13 guards defending Zmiinyi (Snake) Island were among the 137 civilians and military personnel killed in Thursday's attacks, according to Ukrainian President Volodymyr Zelenskyy.

In an audio clip shared online, a soldier from a Russian military ship can be heard telling the Ukrainian guards to "lay down your arms immediately to avoid bloodshed and unjustified deaths," adding "otherwise you will be bombed."

After a brief discussion among themselves, the guards respond by saying: "Russian warship, go f*** yourself."

Ukraine lost contact with its forces defending the island on Thursday after Russia conducted strikes from the air and sea, officials in Kyiv said.

The recording of the exchange between Ukrainian and Russian forces has been circulated by a Ukrainian official and the media outlet Ukrainskaya Pravda.

Anton Herashchenko, an adviser to the interior minister, shared the clip on his Facebook page and said a Russian ship fired missile artillery after border guards refused to surrender.

President Zelenskyy confirmed that all border guards on the Zmiinyi island in the Odessa region had been killed on Thursday.

In a video address, he also said Russia's claim it was only attacking military targets was false and civilian sites had also been struck.

Russian forces launched a full-scale invasion in the early hours of Thursday, which is thought to be Moscow's most aggressive action since the Soviet invasion of Afghanistan in 1979.

By the end of the day, the Ukrainian government said 137 civilians and military personnel had been killed.