Russia has unleashed a string of attacks against Ukrainian rail and fuel facilities, striking crucial infrastructure far from the front line of its eastern offensive.
Meanwhile, two fires were reported at oil facilities in western Russia, not far from the Ukrainian border. It was not clear what caused the blazes.
As both sides in the war brace for what could be a grinding battle of attrition in the country's eastern industrial heartland, top US officials pledged more help to ensure Ukraine prevails.
In a bold visit to Kyiv to meet president Volodymyr Zelenskiy on Sunday, the American secretaries of state and defence said Washington had approved a $165 million (€154m) sale of ammunition – non-US ammo, mainly if not entirely to fit Ukraine's Soviet-era weapons – along with more than $300 million (€280m) in financing to buy more supplies.
US secretary of state Antony Blinken said after the meeting that the West's united support for Ukraine and pressure on Moscow are having "real results".
“When it comes to Russia’s war aims, Russia is failing. Ukraine is succeeding,” he added.
The US promised on Monday to reopen its embassy in Kyiv soon.
In an interview with the Associated Press, Ukrainian foreign minister Dmytro Kuleba welcomed the American support but said that “as long as Russian soldiers put a foot on Ukrainian soil, nothing is enough”.
Mr Kuleba warned that if western powers want Ukraine to win the war and “stop Putin in Ukraine and not to allow him to go further, deeper into Europe”, then countries need to speed up the delivery of the weapons requested by Ukraine.
Russian president Vladimir Putin accused the US and its allies of trying to "split Russian society and to destroy Russia from within".
Anatoly Antonov, Russia’s ambassador to the US, said the arms deliveries were aimed at weakening Russia but that they were escalating the conflict in Ukraine while undermining efforts to reach some sort of peace agreement.
“What the Americans are doing is pouring oil on the flames,” Mr Antonov told the Rossiya 24 TV channel. “I see only an attempt to raise the stakes, to aggravate the situation, to see more losses.”
Mr Antonov, who has served as ambassador to Washington since 2017, said an official diplomatic note had been sent to Washington expressing Russia’s concerns. No reply had been given, Mr Antonov said.
"We stressed the unacceptability of this situation when the United States of America pours weapons into Ukraine, and we demanded an end to this practice," Mr Antonov said.
When Russia invaded on February 24th, its apparent goal was the lightning capture of Kyiv and perhaps the toppling of its government. But the Ukrainians, with the help of western weapons, bogged down Mr Putin’s troops and thwarted their push to Kyiv.
Moscow now says its goal is the capture of the mostly Russian-speaking Donbas region in the east. While both sides said the campaign in the east is under way, Russia has yet to mount an all-out ground offensive and has not achieved any major breakthroughs.
Ukrainian troops holed up in a steel plant in the strategic city of Mariupol are tying down Russian forces and apparently keeping them from being added to the offensive elsewhere in the Donbas.
Over the weekend, Russian forces launched fresh air strikes on the plant in a bid to dislodge the estimated 2,000 fighters. Some 1,000 civilians were also sheltering in the steelworks, and the Russian military pledged to open a humanitarian corridor for them to leave.
The Russian offer was met with scepticism by Ukraine. Deputy prime minister Iryna Vereshchuk said on the Telegram messaging app that Ukraine does not consider the route safe and added that Russia had breached agreements on similar evacuation routes before. She called on the United Nations to oversee an evacuation.
Mariupol has endured fierce fighting since the start of the war because of its strategic location on the Sea of Azov. In addition to freeing up Russian troops, its capture would deprive Ukraine of a vital port and allow Moscow to establish a land corridor to the Crimean peninsula, which it seized from Ukraine in 2014.
On Monday, Russia focused its firepower elsewhere, with missiles and war planes striking far behind the front lines, in an apparent bid to slow the movement of Ukrainian supplies towards the east and disrupt the flow of fuel needed by the country’s forces.
Oleksandr Kamyshin, the head of the state-run Ukrainian Railways, said five railway facilities in central and western Ukraine were hit on Monday. That included a missile attack near the western city of Lviv.
Ukrainian authorities said that at least five people were killed by Russian strikes in the central region of Vynnytsia.
Russia also destroyed an oil refinery in Kremenchuk in central Ukraine, along with fuel depots there, said Russian defence ministry spokesman Maj Gen Igor Konashenkov. In all, Russian warplanes destroyed 56 Ukrainian targets overnight, he said.
Phillips P O’Brien, professor of strategic studies at the University of St Andrews, said the war is, for now, settling into a campaign of incremental battlefield losses and gains.
“The two sides are sort of every day weakening each other,” he said. “So it’s a question of what can you bring in that’s new” and “what can you destroy on the other side”.
Meanwhile, a major fire erupted early on Monday at an oil depot in a Russian city about 60 miles from the Ukrainian border, Russia’s emergencies ministry said. No cause was given for the blaze. Photos showed a huge, churning plume of thick smoke.
The oil depot in Bryansk is owned by a subsidiary of the Russian state company Transneft, which operates the Druzhba pipeline that carries crude west to other European countries. The ministry said the blaze damaged a depot containing diesel fuel. It said the region has enough diesel for 15 days.
It was not clear if the depot was part of the pipeline infrastructure, but Polish pipelines operator Pern said deliveries to Poland have not been affected.
A Russian news report said another oil storage facility in Bryansk also caught fire on Monday.
Last month, two Ukrainian helicopter gunships hit an oil depot in Russia’s Belgorod region, close to the Ukrainian border.
In a video address on Monday, Mr Zelenskiy described his meeting with Mr Blinken and US defence secretary Lloyd Austin as "encouraging and, importantly, effective".
The Ukrainian leader added that they agreed “on further steps to strengthen the armed forces of Ukraine and meet all the priority needs of our army”.
With Russia’s shift in focus towards the Donbas, Mr Zelenskiy is now focused on more heavy weaponry, such as tanks and artillery.
“We want to see Ukraine remain a sovereign country, a democratic country able to protect its sovereign territory,” Mr Austin said. “We want to see Russia weakened to the point where it can’t do things like invade Ukraine.” – AP/Reuters