Ukrainian authorities rejected a Russian ultimatum for Mariupol to surrender.
Russia is guilty of a "massive war crime" in Mariupol and is violating "the laws of war", the EU's top diplomat said on Monday, as the bloc's foreign ministers gathered to discuss additional sanctions against Russia.
"Russia really doing a lot of war crimes," Josep Borrell told reporters ahead of a foreign ministers' meeting in Brussels.
"What's happening in Mariupol is a massive war crime, destroying everything, bombarding and killing everybody in an indiscriminate manner," he said, adding: "The city will be completely destroyed and people are dying."
Ministers were gathering in Brussels to discuss a fifth round of sanctions against Russia over its military invasion of Ukraine, ahead of a European Council summit later in the week in which US President Joe Biden will also take part.
So far the bloc has targeted Russia's Central Bank and other key banks, the steel sector and hundreds of individuals and entities they say are participating directly or indirectly in the military assault. This includes Russian President Vladimir Putin, Foreign Minister Sergei Lavrov and some prominent oligarchs and businesspeople.
Russian troops have encircled the strategic port city of Mariupol for about two weeks with hundreds of thousands of residents now left without running water and electricity.
Ceasefires to allow for the evacuation of civilians and to permit the delivery of humanitarian aid have repeatedly been broken.
Local authorities say more than 2,400 civilians have been killed as Russian shelling hit civilian buildings including a school and a theatre where people were sheltering from the fighting, and that many were buried in mass graves.
They have also rejected a Russian ultimatum to surrender by Monday 05:00 local time.
On the deployment by Russia of new hypersonic weapons, Borrell said: "Russia is using all military capacities. The problem is that using military capacities against civilians".
"It's not a war, it's massive destruction of a country without any kind of consideration for the laws of war because the war has also laws.
"Legally, (the) international court has already condemned the invasion; morally, they (Russia) have lost any kind of ground because what they're doing is completely out of any kind of law that rule war."
"Russia, Putin deserve the strongest condemnation of the civilised world," Borrell also said.
He vowed that the EU "will continue working supporting Ukraine with all our resources" and "continue talking about what kind of sanctions we can think again, more, especially related to energy".
Foreign ministers were also set to discuss the bloc's new "strategic compass", which aims to identify the ways the EU need to adapt to the growing challenges worldwide.
The French presidency of the EU hopes that the compass will be approved by leaders at this week's summit.
"We have been working on that for two years and when we started working we couldn't imagine that at the last moment of approval the situation would be so bad and that Europe was going to face such a big challenge," Borrell said.
"That's the moment to rethink about the future of European capacities to face challenges like war. That's why EU member states will have to increase their military capacities, and do it in a coordinated manner. As responsible for the Common Security and Defence Policy, we will be working in order to make us stronger militarily, and use our capacities in a more coordinated way," he said.