Ukraine's President Volodymyr Zelensky accused Moscow of "madness" Tuesday after Russian troops hit a chemical plant in their bid to complete the capture of a key eastern city.

The battle for control of Severodonetsk has been intensifying this week, with heavy casualties on both sides, as EU leaders haggle over banning Russian gas to punish the Kremlin for its three-month-old invasion of its pro-Western neighbour.

One of the industrial hubs on Russia's path to taking the eastern Lugansk region, Severodonetsk has become a target of massive Russian firepower since the failed attempt to occupy Kyiv.

Russians now control most of the destroyed city, regional authorities said Tuesday, adding that enemy forces had hit a nitric acid tank at a chemical plant and warning people to stay indoors.

"Given the presence of large-scale chemical production in Severodonetsk, the Russian army's strikes there, including blind air bombing, are just crazy," Zelensky said in a video message.

"But on the 97th day of such a war, it is no longer surprising that for the Russian military, for Russian commanders, for Russian soldiers, any madness is absolutely acceptable."

Meanwhile, in Brussels European Union leaders were split over banning gas from Moscow after agreeing to embargo two-thirds of its oil to tighten the economic screws.

These nations played down the chances of a rapid gas ban to follow, but Zelensky nevertheless expressed his gratitude for EU action taken so far against "the terrorist state" of Russia.

"It is also important to understand that European countries' abandonment of Russian oil and other fossil fuels will accelerate the transition to renewable energy sources," he said.

"Strategically, this leaves the Russian state on the sidelines of the modern economy. With such an aggressive policy and a course of isolation from the civilized world, Russia simply will not be able to adapt."

- Constant shelling -

The US State Department also applauded the EU's efforts, saying there was "broad support" among Washington's allies for "cutting off the strength of Russia's war machine."

European Commission chief Ursula von der Leyen suggested Brussels had gone far enough against Russian fossil fuels, however, and that it was time to focus more on the "financial and the economic sector".

The oil ban "will effectively cut around 90 percent of oil imports from Russia to the EU by the end of the year", she said.

Denmark became the latest European country to be targeted by Russia over gas exports in the meantime, following the Netherlands, Finland, Poland and Bulgaria.

Danish energy firm Orsted said Russian monopoly Gazprom Export would cut gas supplies on Wednesday after the Danes refused to pay in rubles.

The situation on the eastern frontline in Donbas has become increasingly desperate, with Ukrainian towns facing near constant shelling from Russian forces.

A steady flow of vehicles delivered some 1,350 people Monday through an eastern Ukrainian checkpoint towards Red Cross buses, desperate to flee the horrors of life in Russian-controlled territory.

The exhausted crowds, including women and children, crossed a dam cutting through a reservoir that serves as the demarcation line between Russian and Ukrainian troops.

"I can breathe more easily now," Anna, a teacher who fled with her children aged 13 and 11, told AFP, as she crossed the checkpoint.

Tetyana, a 19-year-old student, said she was happy to back in her "native Ukraine" after three months of living in "the Russian world".

French journalist Frederic Leclerc-Imhoff was killed while covering civilian evacuations in the Donbas on Monday.

And an overnight rocket attack left at least three people dead and six wounded in the city of Sloviansk, Donetsk regional governor Pavlo Kyrylenko said Tuesday on Telegram.

- 'Save your lives' -

"There are no safe places in the Donetsk region, so I call again: evacuate -- save your lives," he said.

Four more civilians died and seven were injured in Donetsk on Tuesday, he added in a later Telegram post.

Ukraine's prosecutor general Iryna Venediktova said authorities had identified a "few thousand" cases of war crimes in the Donbas, including murder, torture and the forced displacement of children.

The key Zelensky aide, who met international counterparts in The Hague on Tuesday, said Kyiv was already going to prosecute 80 suspects for alleged war crimes on Ukrainian soil.

A Ukrainian court on Tuesday jailed two Russian soldiers for 11 and a half years for shelling two villages in the northeastern Kharkiv region. Earlier this month, another was jailed for life for murdering a civilian.

Russia's invasion of its pro-Western neighbour is also threatening a global food crisis, with Ukraine's huge grain harvest effectively taken off the world market. 

French President Emmanuel Macron said Tuesday he and German Chancellor Olaf Scholz had urged Russian leader Vladimir Putin to end Russia's blockade of the Ukrainian port of Odessa.

But Russian Foreign Minister Sergei Lavrov said it was up to the West and Kyiv to resolve the crisis, starting with the lifting of sanctions.

Ukraine's Foreign Minister Dmytro Kuleba called on Macron to visit before the end of France's EU presidency on June 30.

"It would be good that Macron came during the French EU presidency, and the best thing would be that he comes with more weapons deliveries for Ukraine," he told French news channel LCI.

"That's the most precious aid we can receive from France."

Latest developments

Here are the latest developments in the war in Ukraine:

- Russians seize 'most' of key eastern city -

Russian forces seize control of most of the eastern Ukrainian city of Severodonetsk, a regional governor says. 

"Unfortunately, today, Russian troops control most of the city," Lugansk governor Sergiy Gaiday says in a video. 

The industrial city is key to Russia's goal of capturing the Donbas region, where Moscow has shifted the bulk of its firepower since failing to seize Kyiv.

- EU agrees ban on most Russian oil -

EU leaders agree to ban most Russian oil imports, after reaching a deal with Hungary that allows it to keep receiving Russian crude.

The deal bans oil imports delivered by tankers but exempts pipeline deliveries, a key demand of landlocked Hungary, whose leader Viktor Orban had warned that a full oil blockade would be an "atomic bomb" for his country's economy.

European Commission chief Ursula von der Leyen says the accord will cut around 90 percent of the EU's Russian oil imports by the end of the year.

But EU leaders play down the prospects of getting a ban on Russian gas in a next round of sanctions.

- Gazprom cuts gas supplies -

Danish energy company Orsted says Russian gas giant Gazprom Export will cut supplies to Denmark on June 1 after the Danish company refused to pay in rubles.

Dutch energy firm GasTerra says Gazprom has suspended its gas supplies for the same reason.

Finland, Bulgaria and Poland have already seen their gas supplies severed.

- 'Thousands' of alleged war crimes in Donbas -

Ukraine is investigating several thousand cases of suspected war crimes in the Donbas region, Kyiv's chief prosecutor says.

On a visit to The Hague, Ukraine's prosecutor general Iryna Venediktova says some 15,000 cases of alleged war crimes have been reported countrywide since Russia's invasion on February 24.

- Mariupol port resumes business - 

A Russia-bound cargo ship has left occupied Mariupol, less than two weeks after the last Ukrainian defenders of the strategic port city surrendered, the city's new rulers say.

Dozens of container ships are blocked in Ukrainian ports which have been blockaded by Russian vessels. Russia is in talks with Turkey about creating a secure corridor for shipping companies.

Separately, French President Emmanuel Macron says he and German Chancellor Olaf Scholz have urged Vladimir Putin to end Russia's blockade of the major Ukrainian Black Sea port of Odessa under the terms of a UN resolution.

- 'Banking sanctions hurt Africa'

The chairman of the African Union, Macky Sall, warns EU leaders their decision to expel Russian banks from the SWIFT financial messaging system risks hurting Africa's food supplies by making it harder to import Russian grain.

On Monday, the EU barred Russia's largest bank Sberbank from SWIFT as part of a sixth round of sanctions on the Russian economy.

The move makes it difficult for the bank to receive or make international payments but Sberbank says it is operating "as normal".

Russian Foreign Minister Sergei Lavrov says it was up to the West and Kyiv to resolve the growing global food crisis.

Turkey's top diplomat says Lavrov will arrive in Turkey on June 8 for talks on unblocking grain exports from Ukraine.

- Two more Russian soldiers convicted -

Two Russian soldiers are sentenced to more than 11 years in jail in Ukraine for breaching the rules of war by shelling civilian areas in the northeastern Kharkiv region.

Their convictions come a week after a Kyiv court sentenced a 21-year-old Russian soldier to life in prison for shooting dead an unarmed civilian on his bicycle.