Russian President Vladimir Putin told visiting UN Secretary-General Antonio Guterres that he still hopes for negotiations to end the conflict in Ukraine.

"Despite the fact that the military operation is ongoing, we still hope that we will be able to reach agreements on the diplomatic track," Putin told Guterres in televised remarks after their meeting at the Kremlin on April 26.

Putin noted that Russian and Ukrainian negotiators made what he described as a “serious breakthrough” in their talks in Istanbul last month. But he said that efforts had been derailed by claims of atrocities committed by Russian forces in the town of Bucha, outside Kyiv.

"There was a provocation in the village of Bucha, which the Russian Army had nothing to do with," Putin said.

Putin also agreed "in principle" to UN and Red Cross involvement in the evacuation of civilians from the Azovstal plant in Ukraine's Mariupol, the United Nations said in a statement.

"Follow-on discussions will be had with the United Nations Office for the Coordination of Humanitarian Affairs and the Russian Defense Ministry," UN spokesman Stephane Dujarric said in a statement after Putin’s meeting with Guterres.

Recent attempts to set up humanitarian corridors for the evacuation of civilians have failed.

During the meeting, Guterres criticized Russia’s military action in Ukraine as a flagrant violation of its neighbor’s territorial integrity. He also urged Russia to allow the evacuation of civilians trapped in the steel mill.

Putin responded by claiming that the Russian forces had offered humanitarian corridors but said the Ukrainian defenders of the plant were using civilians as shields and not allowing them to leave.

Before meeting the Russian president, Guterres had earlier talked with Foreign Minister Sergei Lavrov and told him he was concerned about reports of possible war crimes in Ukraine and that independent investigations are needed.

He also said that the United Nations was ready to fully mobilize the world body's resources to save lives and evacuate people from the besieged Ukrainian city of Mariupol.

The sprawling steel complex at Azovstal has remained the last bulwark of Ukrainian resistance in the strategic port on the Sea of Azov. Ukraine's Defense Ministry said Russia continued to attack the Azovstal plant.

Ukrainian officials have said that up to 1,000 civilians have sheltered in the maze of underground tunnels there. They have repeatedly urged Russia to offer them a safe exit.

Meanwhile, as Russian forces pounded eastern Ukraine on April 26, U.S. Defense Secretary Lloyd Austin promised to get Kyiv the weapons it needs to repel the new offensive.

Austin said that more help for Ukraine was on the way, as he convened a meeting of officials from around 40 countries at the U.S. Ramstein Air Base in Germany to pledge more weapons.

"This gathering reflects the galvanized world," Austin said in his opening remarks. He added that he wanted officials to leave the meeting “with a common and transparent understanding of Ukraine’s near-term security requirements because we’re going to keep moving heaven and earth so that we can meet them.”

Austin also praised a decision by Germany, which said it had cleared the way for delivery of Gepard antiaircraft guns to Ukraine.

U.S. Secretary of State Antony Blinken said Putin has shown no seriousness about diplomacy to end the war despite a series of international efforts.

"We've seen no sign to date that President Putin is serious about meaningful negotiations," Blinken told the Senate Foreign Relations Committee on April 26.

He noted that in talks with Russia ahead of the February 24 invasion it became clear that Putin's complaints about Ukraine entering NATO were only a pretext for the invasion.

"It is abundantly clear -- in President Putin's own words -- that this was never about Ukraine being potentially part of NATO and it was always about his belief that Ukraine does not deserve to be a sovereign, independent country," Blinken said.

Fighting continued on April 26 in eastern Ukraine, where the city of Kreminna in the Luhansk region has reportedly fallen to the Russians.

Street-to-street fighting had been going on for days in Kreminna, with civilian evacuations there made impossible by the war, the British Ministry of Defense said on Twitter on April 26.

The British report said heavy fighting was underway in the south of the city of Izyum, as Russian forces try to advance toward the cities of Slovyansk and Kramatorsk in the Donetsk region.

"Ukrainian forces have been preparing defenses in Zaporizhzhya in preparation for a potential Russian attack from the south," it added in the regular bulletin, also saying that Russian troops were likely to attempt to encircle Ukrainian Army's heavily fortified positions in the east.

On the battlefield, Russia's Defense Ministry said on April 26 that its forces had "liberated" the entire Kherson region in southern Ukraine and parts of the Zaporizhzhya, Mykolayiv, and Kharkiv regions, Interfax news agency reported. The claims could not be independently confirmed.

Austin -- who visited Ukraine with Blinken over the weekend and announced an extra $700 million in military aid to Ukraine -- said Washington wants to see "Russia weakened to the degree that it can't do the kinds of things that it has done in invading Ukraine," adding that Kyiv can still win the war if given the right support.

The United States has sent some $4 billion in military aid since President Joe Biden's term began last year, and already announced a new $800 million aid package last week.

Lavrov said late on April 25 that Western arms shipments to Ukraine mean that NATO is essentially engaging in war with Russia through a proxy and arming that proxy, and weapons delivered to Ukraine from the West will be "legitimate targets" for the Russian military.

Lavrov also warned of the threat of a third world war and said there was a "considerable" risk of the conflict escalating to nuclear weapons.

"The danger is serious, it is real, it must not be underestimated," Lavrov said.

But Ukrainian Foreign Minister Dmytro Kuleba said Lavrov's comments are meant to just "scare the world off supporting Ukraine."

Earlier, Russia's ambassador in Washington told the United States to halt arms shipments, warning that large Western deliveries of weapons were inflaming the conflict.

In other developments, the president of Moldova said several alleged attacks in the Moscow-backed breakaway region of Transdniester were an attempt to escalate tensions. President Maia Sandu blamed “pro-war factions" within the territory's administration.

Poland and Bulgaria said Russian state energy giant Gazprom informed them that it will halt the delivery of natural gas supplies beginning on April 27. Both countries said they had enough supplies for the time being.

Putin previously threatened to cut off gas supplies to countries that he called "unfriendly" if they failed to comply with his demand that they pay for gas imports in rubles instead of euros or dollars.

Andriy Yermak, an aide to Zelenskiy, said the move was the beginning of Russia’s “gas blackmail of Europe."