The 13 border guards had been defending the remote Zmiinyi (Snake) Island, a largely uninhabited but strategically important strip of land in the Black Sea, about 186 miles (300km) west of Crimea when a Russian vessel approached on Thursday.
Russian soldiers onboard the vessel told the guards to lay down their weapons or be “bombed”, according to audio clips of the incident picked up by local media and shared online.
Two Ukrainian guards can be heard engaging in a brief exchange, saying, “Should I tell them to go f*** themselves?” – before responding with: “Russian warship, go f*** yourself.”
An earlier statement by Ukraine suggested they were then killed by air and sea strikes - which Russia denied, saying the troops surrendered. Ukraine’s interior ministry said the audio recording was authentic, adding that the presumed-dead soldiers would be given military honours.
However, on Saturday the State Border Guard Service of Ukraine (SBGSU) said: “We [have a] strong belief that all Ukrainian defenders of Zmiinyi (Snake) Island may be alive.
“Preliminary information that border guards may be dead came before the defenders lost contact.
“We sincerely hope that the boys will return home as soon as possible. Glory to Ukrainian defenders!”
After the initial reports of soldiers being killed on the island emerged, Ukraine’s president Volodymyr Zelensky said: "On our Zmiinyi Island, defending it to the last, all the border guards died heroically.”
Fierce fighting continues in several cities across Ukraine on day four of the war, announced by Vladimir Putin, the Russian president, early on Thursday morning.
Capital Kyiv has been under sustained attack since Friday morning, although Ukrainians are putting up strong resistance and slowing the Russian advance.
An adviser to Mr Zelenskiy said on Sunday morning that the situation has not changed significantly in Ukraine, with the country in control of the territory west of Kyiv and Russian troops not making any advance in the south.
"The fact that a Russian delegation arrived in Gomel with a proposal to conduct peace talks from a military point of view means only one thing: they have lost the initiative and are now unable to continue active hostilities," Oleksiy Arestovych said.