Ukraine called for new sanctions on Russia and highlighted the consequences of catastrophe at Europe's biggest nuclear plant, where fresh shelling nearby has renewed a blame game between both sides.
The world nuclear watchdog has said the world risks a disaster if the fighting does not stop. Ukrainian and Russian-installed officials have traded accusations over who is responsible for attacks close to the Zaporizhzhia nuclear plant in southern Ukraine.
Ukrainian President Volodymyr Zelenskiy has warned Russian soldiers that if they attack the site in the now Russian-controlled city of Enerhodar, or use it as a base to shoot from, then they will become a "special target".
"If through Russia's actions a catastrophe occurs the consequences could hit those who for the moment are silent," he said in a late Monday-night address, calling for new sanctions on Russia's nuclear sector.
"If now the world does not show strength and decisiveness to defend one nuclear power station, it will mean that the world has lost."
Vladimir Rogov, a Russia-installed official in Enerhodar, said on Monday about 25 heavy artillery strikes from US-made M777 howitzers had hit near the nuclear plant and residential areas in a two-hour period.
Russia's Interfax news agency, quoting the press service of Enerhodar's Russian-appointed administration, said Ukrainian forces had opened fire, with blasts near the power plant.
But according to the head of the administration of the Nikopol district, which lies across the river from Enerhodar and remains under Ukrainian control, it was Russian forces that had shelled the city to try to make it appear that Ukraine was attacking it.
"The Russians think they can force the world to comply with their conditions by shelling the Zaporizhzhia NPP (nuclear power plant)," Andriy Yermak, chief of the Ukrainian presidential staff, wrote on Twitter.
Reuters could not immediately verify battlefield reports.
The United Nations says it has the logistics and security capacity to support a visit by the International Atomic Energy Agency (IAEA) if both Russia and Ukraine agree.
Russian Defence Minister Sergei Shoigu held a phone call with Guterres to discuss conditions for the safe functioning of the plant, the ministry said on Monday.
"In close cooperation with the agency and its leadership, we will do everything necessary for the IAEA specialists to be at the station and give a truthful assessment of the destructive actions of the Ukrainian side," Russia's foreign ministry spokesperson Maria Zakharova said.
But Igor Vishnevetsky, deputy head of the foreign ministry's nuclear proliferation and arms control department, was later quoted as saying it would be too dangerous for any IAEA mission to travel through the capital Kyiv to inspect the plant.
"Imagine what it means to pass through Kyiv - it means they get to the nuclear plant through the front line," RIA news agency quoted Vishnevetsky.
Ukraine, where parliament on Monday extended martial law for a further three months, has said for weeks it is planning a counteroffensive to recapture Zaporizhzhia and neighbouring Kherson province, the largest part of the territory Russia seized after its February 24 invasion and still holds.
The conflict, which has caused millions to flee and killed thousands, has put major strain on relations between Moscow and the West.
A Russian-backed separatist court in eastern Ukraine's Donetsk charged five foreign nationals it said were captured fighting with Ukrainian forces with being mercenaries on Monday, Russian media reported. Three of the men could face the death penalty.
Russia late on Monday said British reconnaissance aircraft violated its air border at a peninsula east of Finland between the Barents Sea and the White Sea, and a fighter jet forced the British aircraft out of Russian airspace.
Britain's defence ministry did not immediately respond to a request for comment.
Russia calls its invasion of Ukraine a "special military operation" to demilitarise its neighbour and protect Russian-speaking communities. Ukraine and Western backers accuse Moscow of waging an imperial-style war of conquest.
Russian forces were engaged in shelling to advance on a wide variety of frontline positions in the east and south, Ukrainian forces reported on Monday evening.
Even as the biggest attack on a European state since 1945 ground on, there was progress on a grain deal to ease a global food crisis created by the conflict, the most significant diplomatic breakthrough achieved since the war began.
The Joint Coordination Centre, set up by the United Nations, Russia, Ukraine and Turkey, said it had approved the departure of the Brave Commander, the first humanitarian food aid cargo bound for Africa from Ukraine since the invasion. It is set to leave on Tuesday.