Russia’s top diplomat warned Ukraine against provoking World War III and said the threat of a nuclear conflict “should not be underestimated” as his country unleashed attacks against rail and fuel installations far from the front lines of Moscow’s new eastern offensive.

Meanwhile, the British Defense Ministry said Tuesday that Russian forces had taken the Ukrainian city of Kreminna in the Luhansk region after days of street-to-street fighting.

“The city of Kreminna has reportedly fallen and heavy fighting is reported south of Izium as Russian forces attempt to advance towards the cities of Sloviansk and Kramatorsk from the north and east,” the British military said in a tweet. It did not say how it knew the city, 575 kilometers (355 miles) southeast of the Ukrainian capital, Kyiv, had fallen. The Ukrainian government did not immediately comment.

Ukraine’s General Staff said Russian forces were shelling Kharkiv, the country’s second-largest city, as they fought to take full control of the Donetsk and Luhansk regions, which comprise the Donbas in Ukraine’s industrial heartland, and establish a land corridor to Crimea.

In the area of Velyka Oleksandrivka, a village in the Kherson region largely controlled by Russians, Ukrainian forces destroyed an ammunition depot and “eliminated” more than 70 Russian troops, the General Staff said.

The governor of the Luhansk region, Serhiy Haidai, said on the messaging app Telegram that the Russians had shelled civilians 17 times over the previous 24 hours, with the cities of Popasna, Lysychansk and Girske suffering the most.

Four people died and nine more were wounded on Monday in the Russian shelling of the Donetsk region, its governor Pavlo Kyrylenko said on Telegram. He said a 9-year-old girl and a 14-year-old boy were among those killed.

The U.S. has been rushing more weaponry to Ukraine and said the assistance from Western allies is making a difference in the 2-month-old war.

“Russia is failing. Ukraine is succeeding,” U.S. Secretary of State Antony Blinken declared Monday after he and the U.S. secretary of defense made a bold visit to Kyiv to meet with President Volodymyr Zelenskyy.

Blinken said Washington approved a $165 million sale of ammunition — non-U.S. ammo, mainly if not entirely for Ukraine’s Soviet-era weapons — and will also provide more than $300 million in financing to buy more supplies.

U.S. Defense Secretary Lloyd Austin went further, saying the U.S. wants to see Ukraine remain a sovereign, democratic country, but also wants “to see Russia weakened to the point where it can’t do things like invade Ukraine.”

Austin’s remarks appeared to represent a shift in U.S. strategic goals since earlier Washington said the goal of American military aid was to help Ukraine win and to defend Ukraine’s NATO neighbors against Russian threats.

In an apparent response to Austin, Russian Foreign Minister Sergei Lavrov said Russia has “a feeling that the West wants Ukraine to continue to fight and, as it seems to them, wear out, exhaust the Russian army and the Russian military industrial war complex. This is an illusion.”

Weapons supplied by Western countries “will be a legitimate target,” said Lavrov, who accused Ukrainian leaders of provoking Russia by asking NATO to become involved in the conflict. NATO forces are “pouring oil on the fire,” Lavrov said, according to a transcript on the Russian Foreign Ministry’s website.

“Everyone is reciting incantations that in no case can we allow World War III,” he said in a Russian television interview.

Lavrov said he would not want to see risks of a nuclear confrontation “artificially inflated now, when the risks are rather significant.”

“The danger is serious,” he said. “It is real. It should not be underestimated.”

Ukrainian Foreign Minister Dmytro Kuleba said on Twitter that Lavrov’s comments underscore Ukraine’s need for Western help: “Russia loses last hope to scare the world off supporting Ukraine. Thus the talk of a ‘real’ danger of WWIII. This only means Moscow senses defeat in Ukraine.”

When Russia invaded Ukraine on Feb. 24, its apparent goal was to seize Kyiv, the capital. But the Ukrainians, helped by Western weapons, forced President Vladimir Putin’s troops to retreat.

Moscow now says its goal is to take the Donbas, the mostly Russian-speaking industrial region in eastern Ukraine, where residents are struggling to survive without many of the basics, collecting rainwater for cleaning and washing up and fervently hoping for an end to the fighting.

“When you open a plastic bottle and it makes a crackling sound, you are worried at once (thinking that it’s an explosion) because of all those blasts. Anything that is happening, any noise, if our neighbours bang the door, a metal door, you are startled,” said Andriy Cheromushkin, a resident of Toretsk, a small city south of Kramatorsk.

“It’s bad. Very bad. Hopeless,” he said. “You feel so helpless that you don’t know what you should do or shouldn’t do. Because if you want to do something, you need some money; and there is no money now.”

On Monday, Russia was focusing its firepower beyond the Donbas, with missiles and warplanes striking far behind the front lines to try to thwart Ukrainian supply efforts.

Five railroad stations in central and western Ukraine were hit, and one worker was killed, said Oleksandr Kamyshin, head of Ukraine’s state railway. Missiles struck Lviv, the western city near the Polish border jammed by Ukrainians fleeing their home.

Ukrainian authorities said at least five people were killed by Russian strikes in the central Vynnytsia region.

Russia also destroyed an oil refinery and fuel depots in Kremenchuk, in central Ukraine, Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said. In all, Russian warplanes destroyed 56 Ukrainian targets, he said.

The strikes on fuel depots are meant to deplete vital Ukrainian war resources. Strikes against rail targets, both disrupt supply lines and intimidate people trying to use the railways to flee the fighting, said Philip Breedlove, a retired U.S. general who was NATO’s top commander from 2013- 2016.

An estimated 2,000 Ukrainian troops holed up in a steel plant in the strategic southern port city of Mariupol are tying down Russian forces, apparently preventing them from joining the offensive elsewhere in the Donbas. Over the weekend, Russian forces launched new airstrikes on the Azovstal plant to try to dislodge the holdouts.

Some 1,000 civilians were also said to be taking shelter at the steelworks.

The city council and mayor of Mariupol said a new mass grave was identified about 10 kilometers (6 miles) north of the city. Mayor Vadym Boychenko said authorities were trying to estimate the number of victims. It was at least the third new mass grave discovered in Russian-controlled areas near Mariupol in the last week.

Mariupol has been gutted by bombardment and fierce street fighting over the past two months. Russia’s capture of the city would deprive Ukraine of a vital port and give Moscow a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014.

In his nightly video address, Zelenskyy said Ukraine was maintaining its resistance to “make the occupiers’ stay in our land even more intolerable,” while Russia drains its resources.

Britain said it believes 15,000 Russian troops have been killed in Ukraine since Russia’s invasion began. Defense Secretary Ben Wallace said 25% of the Russian combat units sent to Ukraine “have been rendered not combat effective.”

Ukrainian officials have said about 2,500 to 3,000 Ukrainian troops had been killed as of mid-April.

AP