Why is Mariupol key to Russia's invasion plan?
For more than three weeks, the Ukrainian port city of Mariupol has become the most heavily bombed and damaged city in Ukraine since Russia launched its invasion.
Ukrainian President Volodymyr Zelensky estimated that 100,000 civilians remain in Mariupol, the scene of some of the war's worst devastation.
The question is why is the port city, on the Sea of Azov, so crucial to Russia's military campaign in Ukraine?
The importance of the port city
Mariupol may only occupy a tiny area on the map, but it importantly stands resolutely in the way of Russian forces who have burst out of the Crimean peninsula.
It is not only a large port city but a base for Ukrainian armed forces.
Most importantly, if the Russians want to have a land corridor from the Donbas (southern-eastern Ukraine) to Crimea, they need to take control of the city.
Historian and author Professor Margaret MacMillan explained Mariupol's significance to Putin's plan.
"I think it's partly to weaken Ukraine. Ports are very important to Ukraine. Ukraine is a great exporting nation.
"I think what Putin has always wanted is a land bridge, from Crimea, across the southern part of Ukraine.
"If he can get Mariupol, I think he has much more capacity to choke Ukraine and so even a neutral Ukraine... would be heavily dependent on Russian goodwill to export," she said.
Reports say if Mariupol were to be seized, Russia could end up with full control of more than 80% of Ukraine's Black Sea coastline, potentially cutting off its maritime trade and further isolation from the world.
MacMillan added: "It is an extraordinary feat that the people of Mariupol and the forces are managing to hold out in these appalling circumstances.
"Medicine is disappearing, food is disappearing and yet, they're hanging on, and I think for Putin it is, I think, a tactical goal, but I think it's also a symbolic goal."
Putin's goals versus Ukrainian nationhood
It is difficult to truly understand the goals of President Putin, but for a number of years, reports have come to the consensus that he regards Ukraine as part of Russia.
MacMillan says: "He thinks it's quite illegitimate that Ukraine should be an independent state and he refuses to accept that there's anything called Ukrainian nationhood."
However, she added: "In fact, one of the ironies, many ironies, about this is he's probably done more to create a sense of Ukrainian nationhood than any ruler of Russia."
The historian believes "it's going to be very difficult for him (Putin) now to back down" partly due to his big goals but because "he faces humiliation" because of the poor performance of his troops.
She said: "The performance of his armed forces has not been what he confidently expected. I mean, we now know that captured luggage of Russian officers had their dress uniforms in it.
"Putin seems to believe that it will be a victorious march through an acquiescent Ukraine.
"The whole thing will be over in two days.
"Now we're getting close to four weeks and the Russian army has taken, is continuing to take, really dreadful losses, as are the other forces, the navy, and the air force as well."
Watch: Is Ukraine showing early signs of winning war against Russia?
The psychological significance of Mariupol
Capturing Mariupol would be a huge win for Kremlin propaganda, which reports say has portrayed Ukraine as being governed by Nazis and this war as a "de-Nazification".
Mariupol is home to a Ukrainian militia unit called the Azov Brigade, named after the Sea of Azov which links Mariupol to the rest of the Black Sea.
This brigade contains far-right extremists, historically including roots in far-right and neo-Nazi groups.
Although they only form the tiniest fraction of Ukraine's national guard, Russian propaganda has claimed Azov fighters were responsible for the killings of civilians and destruction in Mariupol.
Capturing significant numbers of Azov Brigade fighters would more than likely be paraded on Russian state-controlled media as part of the ongoing information war to discredit Ukraine and its government.
Mariupol would be the first major city to fall to the Russians after Kherson, which in contrast did not put up a strong resistance as Mariupol so the moral implications could be major.
Historian MacMillan said it would be a "big blow" to Ukraine if they were to lose Mariupol but believes she did not "see any indication that they would want to stop fighting".
She said: "Even if Russia manages to occupy a great deal of the country, I think the Russian forces are going to find themselves constantly fighting guerrilla warfare, and both urban and rural, against the people who have every reason to hate them passionately."
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