The United States and EU announced Friday a new drive to wean Europe off Russian gas imports and so choke off the billions in revenues that are fueling Moscow's ruinous war against Ukraine.

A clearer scale of the ruin emerged from Ukraine's besieged port city of Mariupol, which a month into the invasion now resembles scenes of Russian cities razed by the Nazis in World War II.

Authorities said some 300 civilians may have died in a Russian air strike on a theatre-turned-bomb shelter last week, in what would be the invasion's single bloodiest attack.

After a trio of summits in Brussels, US President Joe Biden warned that NATO would "respond" if Russia's Vladimir Putin resorts to chemical weapons as part of his aggression against a Western-leaning democracy.

"The nature of the response would depend on the nature of the use," Biden said.

A joint energy task force was announced by Biden and EU commission chief Ursula von der Leyen in Brussels, before he headed to the eastern Polish town of Rzeszow, a mere 80 kilometers (50 miles) from Ukraine.

Taken together, Western sanctions are "draining Putin's resources to finance this atrocious war", von der Leyen told reporters alongside Biden.

On the battlefield, Moscow said it had destroyed Ukraine's largest remaining military fuel depot, at Kalynivka near the capital Kyiv, using sea-borne cruise missiles.

Fireballs

Fireballs leapt into the air from the storage facility, while a smaller fire blazed from a severed fuel line and a huge plume of black smoke rose over the site, AFP reporters at the scene said.

"Fortunately, there were no casualties," a security guard said at a checkpoint near the depot, asking not to be identified.

In the east, Russian strikes targeting a medical facility in Ukraine's second city Kharkiv killed four civilians and wounded several others, police said.

NATO, European Union and G7 leaders in Brussels shied away from impassioned demands by Ukrainian President Volodymyr Zelensky for more advanced weapons systems to take the fight to the Russian invaders.

He underlined the toll inflicted by incessant Russian bombardment of cities such as the southern port of Mariupol -- where authorities said a horrifying picture was emerging from the Drama Theatre.

Up to 1,000 civilians were said to be sheltering in the theatre when it was flattened by a Russian bomb last week. Ukraine said efforts to dig people out of the ruins were hampered by relentless bombardment.

"From eyewitnesses, information is emerging that about 300 people died in the Drama Theatre of Mariupol following strikes by a Russian aircraft," Mariupol city hall wrote on Telegram.

Zelensky says nearly 100,000 people are trapped without food, water or power in the besieged city.

Time to choose

Addressing the EU summit late Thursday by video feed, he thanked countries including Poland and Estonia for their support, noted German backing came "a little later" -- and singled Hungary out for censure.

"You have to decide for yourself who you are with," Zelensky told Hungary's right-wing populist leader Viktor Orban, who has close ties to Moscow.

"Listen, Viktor, do you know what's going on in Mariupol?" he added.

The city is a treasured prize for Russia as it would enable a land bridge between Russian-annexed Crimea and regions already controlled by Russian proxy forces in eastern Ukraine.

While some civilians have been able to flee to Ukrainian-controlled territory, local officials said as many as 15,000 Mariupol residents have been forcibly deported to Russia.

But while Mariupol is now a charred ruin, Western defensive systems including shoulder-fired anti-tank missiles have helped Ukraine's armed forces hold their line.

"Ukrainian counter-attacks, and Russian forces falling back on overextended supply lines, has allowed Ukraine to re-occupy towns and defensive positions up to 35 kilometers (22 miles) east of Kyiv," Britain's defense ministry said in a daily update.

After several rounds of sanctions banished Russia from much of Western finance, Ukraine's EU allies broadened their economic offensive to energy, which largely powers European homes and industry.

Turning off the gas

Biden and von der Leyen said the United States would strive to help supply Europe with an extra 15 billion cubic meters of liquefied natural gas this year -- replacing one-third of supplies from Russia.

Germany, Moscow's biggest customer in Europe, said it would halve Russian oil imports by June and end all coal deliveries by the autumn.

"The first important milestones have been reached to free us from the grip of Russian imports," Economy Minister Robert Habeck said.

In Poland, Biden will meet members of the US 82nd Airborne Division, part of NATO's increasingly muscular deployment to its eastern flank.

He will also receive a briefing on the dire humanitarian situation in Ukraine, which has lost nearly 3.7 million people, mostly fleeing to Poland.

The UN believes that more than half of Ukraine's children have already been driven from their homes, "a grim milestone that could have lasting consequences for generations to come", according to Unicef chief Catherine Russell.

"Every day it's 20, 30 times we go to the basement (to shelter)," said a sobbing 37-year-old Vasiliy Kravchuk in the garrison town of Zhytomyr.

"It's difficult because my wife is pregnant, I have a little son."