UN Secretary-General Antonio Guterres told reporters after talks in Lviv, Ukraine, on Thursday he was gravely concerned about circumstances at the Zaporizhzhia nuclear plant and called for military equipment and personnel to be withdrawn.
Turkish President Recep Tayyip Erdogan said he, Guterres and Ukrainian President Volodymyr Zelenskiy discussed building on a recent positive atmosphere to revive peace negotiations with Russia that took place in Istanbul in March.
In a deal brokered by the United Nations and Turkey, Russia and Ukraine reached an agreement in July for Russia to lift a blockade of Ukrainian grain shipments, and exports resumed at the beginning of August.
NATO member Turkey has maintained good relations with Russia, an important trade partner, and sought to mediate in the conflict, which began six months ago when Russian forces invaded neighbouring Ukraine.
"Personally, I maintain my belief that the war will ultimately end at the negotiating table. Mr Zelenskiy and Mr Guterres have the same opinion in this regard," Erdogan said.
There was no immediate comment from Moscow.
At the same time, US President Joe Biden's administration is readying about $US800 million ($A1.2 billion) of additional military aid to Ukraine and could announce it as soon as Friday, three sources familiar with the matter said.
Meanwhile, 17 people were killed and 42 wounded in two separate Russian attacks on the northeastern Ukrainian city of Kharkiv, the regional governor said.
At least four explosions hit near a major Russian military airport on the Moscow-controlled Crimean peninsula on Thursday, three sources said. The sources said the explosions were near Russia's Belbek military airport, north of the Black Sea fleet's headquarters in Sevastopol.
Ukraine has hinted it orchestrated other blasts during the past 10 days at other Russian installations in Crimea, which Russia annexed in 2014.
Sevastopol governor Mikhail Razvozhayev, writing on Telegram, said Russian anti-aircraft forces downed a Ukrainian drone and no damage occurred.
The general staff of the Ukrainian armed forces said on Friday that Russian forces had launched attacks and tried to make advances on three or more fronts and Ukrainian forces had repelled them.
In the south, 73 Russian soldiers were killed and 13 pieces of equipment and ammunition depots were destroyed as Ukrainian forces sought to take control of new territory, the regional command said on Facebook.
Russia says its aim in Ukraine is to demilitarise the country and protect Russian-speakers on land that President Vladimir Putin says historically belongs to Russia. Ukraine and the West call it an unprovoked war of conquest.
Guterres reiterated calls for demilitarisation around the nuclear plant.
"Agreement is urgently needed to re-establish Zaporizhzhia's purely civilian infrastructure and to ensure the safety of the area," Guterres said.
Russia, which captured the plant in southern Ukraine soon after the February 24 invasion, said it could shut it down, which Ukraine warned would increase the risk of a nuclear catastrophe.
Russia had earlier rejected as "unacceptable" international calls for a demilitarised zone. Ukrainian engineers are still operating the plant despite the Russian occupation.
The power station sits on the Russian-controlled south bank of a huge reservoir in Enerhodar; Ukrainian forces hold the north bank. Russia and Ukraine traded accusations through the night of shelling civilian areas near the power station, as they have done for days.
Ukraine also accuses Russia of using the plant as a shield for its forces to launch strikes across the reservoir on Ukrainian-held cities, which Russia denies.
Zelenskiy said after meeting Guterres that they had agreed parameters for a possible mission to the plant by the UN nuclear watchdog, the International Atomic Energy Agency.
"Russia should immediately and unconditionally withdraw its forces from the territory of Zaporizhzhia nuclear power plant, as well as stopping any provocations and shelling," Zelenskiy said.
