Ukraine's Azov Battalion is still holding out in Azovstal, a huge steelworks overlooking the Azov Sea.
Justin Crump, a military expert at security consultancy Sybilline, told the BBC there could be 500 to 800 Ukrainian troops holding out in the city.
"[The steelworks] has nuclear bunkers, tunnels, it's built to survive a nuclear conflict - they are really well set for defence," he said.
"They have had more than 50 days to fortify it and build escape routes," Mr Crump added. "I suspect that unless they are wiped out they'll be there a long time. It's credible there would be guerrilla resistance."