United Nations leader Antonio Guterres pleaded on Thursday for a "just peace" in Ukraine, calling in a speech in front of Russian President Vladimir Putin for an end to more than two years of fighting.
Speaking at a BRICS summit hosted by Putin in Russia's Kazan, Guterres also demanded an "immediate ceasefire" in Gaza, the release of hostages and an "immediate cessation of hostilities" in Lebanon.
The UN secretary general was in Russia for the first time since April 2022 and was due to hold talks on Ukraine with Putin later on Thursday.
He spoke on the final day of the BRICS summit, a forum Moscow hopes will help forge a united front of emerging economies against the West.
Guterres has been critical of Moscow's offensive and military annexation of Ukrainian territory but there has been no halt in the fighting, now grinding through its third year.
"We need peace in Ukraine. A just peace in line with the UN Charter, international law and (UN) General Assembly resolution," Guterres told the summit.
He said it was essential to respect "the principles of sovereignty, territorial integrity and political independence of all states".
Other world leaders at the summit also called for an end to Israel's wars in Lebanon and Gaza, with Palestinian president Mahmud Abbas saying Israel was trying to starve Gazan civilians and drive them from their territory.
Putin, meanwhile, said Russia could not be defeated on the battlefield in Ukraine, warning of "illusionary" Western hopes.
Moscow's opponents "do not conceal their aim to deal our country a strategic defeat", he said.
"I will say directly that these are illusionary calculations, that can be made only by those who do not know Russia's history."
Guterres and Putin -- who last met in the Kremlin in April 2022 during Russia's siege of the Ukrainian city of Mariupol -- were due to hold face-to-face talks later on Thursday.
The meeting comes as the United States has queried why thousands of North Korean soldiers have been sent to Russia, speculating that they could be deployed to fight in Ukraine.
Ukraine has strongly criticised Guterres's decision to meet "criminal Putin."
Guterres's spokesman said the UN chief was willing to offer his mediation when both parties were ready and would use the meeting to reiterate his position on the conflict.
The UN secretary general has condemned Moscow's military offensive and visited places in Ukraine where Russia's army have been accused of atrocities.
He also helped broker a deal that allowed Kyiv to safely export grain from its ports through the Black Sea, the most significant diplomatic agreement between Russia and Ukraine amid the conflict.
But there has been little direct contact between the two countries since and Moscow has been advancing in eastern Ukraine for months.
Putin has demanded Kyiv surrender territory as a precondition for a ceasefire.
Guterres and Putin will meet a day after the United States said it believed "thousands" of North Korean soldiers were being trained in Russia.
"We don't know what their mission will be or if they'll go on to fight in Ukraine," a senior US official said.
Putin has not yet commented on the reports.
Russia's foreign ministry spokeswoman said on Wednesday people should "ask Pyongyang" about its troop movements, refusing to confirm or deny the allegations.
As the war in Ukraine dragged on towards its third anniversary, the Russian parliament approved on Thursday a budget that will see defence spending surge by almost 30 percent.
At the BRICS summit, world leaders also called for peace in Gaza and Lebanon.
Chinese President Xi Jinping warned about "serious challenges" in the world and said he hoped BRICS countries could be a "stabilising force for peace".
"We need to continue to push for a ceasefire in Gaza, relaunch the two-state solution and stop the spread of war in Lebanon. There should be no more suffering and destruction in Palestine and Lebanon," Xi said.
Iranian President Masoud Pezeshkian slammed the UN Security Council's role in front of Guterres, saying international bodies "lack the necessary efficiency to extinguish the fire of this crisis".
And Palestinian president Mahmud Abbas said Israel aims to "empty" the Gaza Strip of Palestinians.
Putin also faces calls by the likes of Indian Prime Minister Narendra Modi and South African President Cyril Ramaphosa for fighting to end in Ukraine.
Starting in 2009 with four members -- Brazil, Russia, India and China -- BRICS has expanded to include other emerging nations, including South Africa, Egypt and Iran.
NATO member Turkey said last month it had asked to join the group and President Recep Tayyip Erdogan told delegates Thursday that "we are determined to further our dialogue with the BRICS family, with whom we have developed close relations based on mutual respect".
