The Russian military said it used long-range missiles to destroy a depot in the western Lviv region of Ukraine where ammunition for Nato-supplied weapons was stored, and the governor of a key eastern city acknowledged that Russian forces are advancing in heavy fighting.

The battle for Sievierodonetsk in Ukraine's eastern Donbas area has become the focus of Russia's offensive in recent weeks.

Russia-backed separatists accused Ukrainian forces of sabotaging an evacuation of civilians from the city's besieged Azot chemical plant, where about 500 civilians and an unknown number of Ukrainian fighters are believed to be sheltering from missile attacks. It wasn't possible to verify that claim.

Russian officials had announced a humanitarian corridor from the Azot plant a day earlier, but said they would take civilians to areas controlled by Russian, not Ukrainian, forces.

Gennady Lamtugov, production director of the Alchevsk Metallurgical Complex, speaks on the phone as Russian troops stand by. Photo / AP

The Ukrainian governor of Luhansk, Serhiy Haidai, told The Associated Press that "heavy fighting in Sievierodonetsk continues today as well". The situation in the city is getting worse, Haidai said, because Russian forces have more manpower and weapons.

"But our military is holding back the enemy from three sides at once," he added.

In the Lviv region near the border with Nato member Poland, Russian forces used high-precision Kalibr missiles to destroy the depot near the town of Zolochiv, Russian Defence Ministry spokesman Igor Konashenkov said. Konashenkov said shells for M777 howitzers, a type supplied by the United States, were stored there. He said four howitzers were destroyed elsewhere and that Russian airstrikes also destroyed Ukrainian "aviation equipment" at a military aerodrome in the southern Mykolaiv region.

Ukrainian officials did not immediately comment on the Zolochiv strike.

While focusing most of their attacks on eastern Ukraine, where they are trying to capture large swaths of territory, Russian forces have also been hitting more specific targets elsewhere, using high-precision missiles to disrupt the international supply of weapons and destroy military infrastructure. Civilian infrastructure has been bombarded as well, even though Russian officials have claimed they're only targeting military facilities.

The latest attacks came as Ukraine keeps up its pressure on Western countries to deliver more arms and as Nato countries pledge more heavy weapons for Ukraine.

In response, President Joe Biden said on Wednesday that the US will send another $1 billion in military aid to Ukraine, the largest single tranche of weapons and equipment since the war began. The aid will include anti-ship missile launchers, howitzers, and more rounds for the High Mobility Artillery Rocket Systems - all key weapons systems that Ukrainian leaders have urgently requested.

In recent days, Ukrainian officials have spoken of the heavy human cost of the war, with Kyiv's forces outgunned and outnumbered in the east.

"The losses, unfortunately, are painful, but we have to hold out," Ukrainian President Volodymyr Zelenskyy said in his nightly video address. "The more losses the enemy suffers there, the less strength it will have to continue the aggression. Therefore, the Donbas is key to determining who will dominate in the coming weeks."

Meanwhile, Dmitry Medvedev, the deputy chair of Russia's Security Council, ominously suggested that Russia appears intent on not just claiming territory but eliminating Ukraine as a nation. In a Telegram post, he wrote that he saw Ukraine wants to receive liquefied natural gas from its "overseas masters" with payment due in two years.

He added: "But there's a question. Who said that in two years, Ukraine will even exist on the map?"

Mykhailo Podolyak, an adviser to Zelenskyy, responded on Twitter: "Ukraine has been and will be. Where will Medvedev be in two years? That's the question."

In other developments:

• Mines are killing people in Ukraine even after the fighting leaves their areas.
  • Nato defence ministers discuss sending more weapons to Ukraine.
  • Germany says a reduced flow of Russian gas appears to be a political move.
  • Russia's economic forum takes place but with fewer participants.
  • French president suggests he will visit Kyiv to show support for Ukraine.

Meanwhile, Turkish Foreign Minister Mevlut Cavusoglu said it may be possible to create secure corridors to transport Ukrainian grain through the Black Sea without the need to clear sea mines near Ukrainian ports.

Turkish Foreign Minister Mevlut Cavusoglumet with his Russian counterpart Sergey Lavrov in Ankara, Turkey. Photo / Russian Foreign Ministry Press Service via AP

Cavusoglu's comments came a week after he discussed with his Russian counterpart Sergey Lavrov a UN plan to open up Odesa and Ukraine's other Black Sea ports to allow millions of tons of grains to be shipped to world markets.

Russia has demanded that Ukraine remove mines from the Black Sea before grain exports can resume by ship. Ukraine rejects the proposal, insisting it would leave its ports vulnerable to Russian attacks.

Cavusoglu told reporters that since the locations of the mines are known, it would be possible to establish "secure corridors" that avoid them. Turkey, Russia, and Ukraine have appointed military officers and set up a telephone hotline to try to overcome hurdles over crop exports.

In Russia, Gazprom announced a reduction in natural gas flows through a key European pipeline for the second day in a row, hours after Germany's vice chancellor said its initial move appeared to be political rather than a result of technical problems.

The state-owned energy giant said on Twitter that deliveries through the Nord Stream 1 pipeline to Germany would be cut again on Thursday, bringing the overall reduction through the undersea pipeline to 60 per cent.

The new cut came a day after Gazprom said it would reduce flows by 40 per cent after Canadian sanctions over the war in Ukraine prevented German partner Siemens Energy from delivering overhauled equipment. It blamed the same issue for the additional reduction.

Gazprom also told Italian gas giant Eni that it would reduce gas through a different pipeline by roughly 15 per cent on Wednesday. The reason for the reduction has not been made clear, and the Italian company said it was monitoring the situation.

The reduced flows follow Russia's previous halt of natural gas supplies to Bulgaria, Poland, Finland, the Netherlands and Denmark as Europe works to reduce its dependence on Russian energy because of the war in Ukraine and sanctions. Gas demand has fallen after the winter heating season, but European utilities are racing to refill storage ahead of next winter.

In Ukraine, a UN delegation investigating war crimes has visited areas of the country that were held by Russian troops and found evidence that could support war crimes allegations.

The delegation chaired by Erik Mose, a Norwegian judge, visited sites including the Kyiv suburbs of Bucha and Irpin, where Ukrainian authorities have accused Russia of mass killings of civilians.

Members of an extraction crew work during an exhumation at a mass grave near Bucha, on the outskirts of Kyiv, Ukraine. Photo / Natacha Pisarenko, AP

"At this stage, we are not in a position to make any factual findings or pronounce ourselves on issues of the legal determination of events," Mose said.

"However, subject to further confirmation, the information received and the visited sites of destruction may support claims that serious violations of international human rights law and international humanitarian law, perhaps reaching war crimes and crimes against humanity, have been committed in the areas," he said.

With Ukrainian and international organisations investigating war crimes cases, Mose expressed concern at the risk of investigations "overlapping" or causing witnesses more trauma by probing the same events repeatedly.

US Defence Secretary Lloyd Austin said the West must step up weapons deliveries to Ukraine and prove its commitment to helping the country's military fight along a 1000km front line in a grinding war of attrition with Russia.

Opening a Nato defence ministers meeting in Brussels on supporting Ukraine, Austin urged more than 45 participating nations to demonstrate "our unwavering determination to get Ukraine the capabilities that it urgently needs to defend itself".

"We must intensify our shared commitment to Ukraine's self-defence, and we must push ourselves even harder to ensure that Ukraine can defend itself, its citizens and its territory," he said.

Russia's war in Ukraine has spread a deadly litter of mines, bombs and other explosives. They are killing civilians, disrupting planting, complicating the rebuilding of homes and villages, and will continue taking lives and limbs long after the fighting stops.

Often, blast victims are farmers and other rural workers with little choice but to use mined roads and plough mined fields, in a country relied on for crops that feed the world.

Vadym Schvydchenko, a 40-year-old driver who hit a tank mine that blew up his truck, said he'll steer clear of dirt tracks for the foreseeable future, although they're sometimes the only route to fields and rural settlements. Mushroom-picking in the woods has also lost its appeal to him.

Vadym Schvydchenko stands next to his truck, destroyed by a mine on a dirt track near Makariv, on the outskirts of Kyiv, Ukraine. Photo / Natacha Pisarenko, AP

"I'm afraid something like this can happen again," he said.

Ukraine is now one of the most mined countries in Europe. The east of the country, fought over with Russia-backed separatists since 2014, was already contaminated by mines even before Russia's February 24 invasion multiplied the scale and complexity of the dangers both there and elsewhere.

Ukraine's State Emergency Service says 300,000km sq - the size of Arizona or Italy - need to be cleared of mines. The ongoing fighting will only expand the area.

— John Leicester & Yuras Karmanau, AP