The Kremlin accused Ukrainian forces Monday of firing on Europe's largest nuclear power plant in occupied Ukraine and warned the alleged attacks could have "catastrophic consequences."
Kyiv said Moscow was responsible and called for the area to be demilitarized, adding that two employees had been wounded in recent attacks.
Fighting continued meanwhile along battle lines stretching across eastern Ukraine, and Russia continued its crackdown on dissent at home.
Each side has blamed the other for the escalation in fighting around the nuclear facility in Zaporizhzhia in southeastern Ukraine, which was captured by Russian forces soon after their late February invasion.
Recent fighting at the plant has prompted the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), to warn of "the very real risk of a nuclear disaster."
Ukraine said the plant should be cleared of Russian forces and called for the establishment of an international security and energy team to oversee the site.
"What needs to be done is to remove occupying forces from the station and to create a de-militarized zone on the territory of the station," said Petro Kotin, president of Ukraine's nuclear energy company, Energoatom.
"There should be a peacekeeping mission including experts from the IAEA and other security organizations," he added.
"Their presence and initially giving control to them and then to the Ukrainian side would have solved this problem."
Kotin said Russia had deployed some 500 Russian troops and 50 pieces of hardware including tanks at Zaporizhzhia, echoing earlier claims from Kyiv that Moscow was using the site for military cover.
Ukraine's Prime Minister Denys Shmygal meanwhile accused Russia of "nuclear terrorism," urging the world to "unite now to prevent a catastrophe."
The Kremlin has captured swathes of Ukrainian territory since February, introducing Russian currency and announcing plans to formally annex the areas by holding votes, including in the Zaporizhzhia region.
The head of its Moscow-installed administration Yevgeny Balitsky said on social media Monday he had signed a decree "on the issue of organizing a referendum on the reunification of the Zaporizhzhia region with the Russian Federation."
The plans follow a similar move by Moscow in 2014 after it annexed the Crimean peninsula, a highly controversial vote that has not been recognized by most Western countries.
Although attention was focused on the fallout from fighting at Zaporizhzhia, Russian forces continued shelling across the sprawling eastern frontline.
Among the attacks, Ukraine said one woman had died and four more were injured in Russian bombardments on Kharkiv region in the northeast. It has been under persistent Russian shelling for weeks.
Ukrainian forces in turn said they had bombed a strategic bridge over the Dnipro river overnight in the Russian-held city of Kherson in the south.
More than five months of fighting has taken a heavy toll on Russian forces, with up to 80,000 of Moscow's troops killed or wounded and 3-4,000 armored vehicles lost, a senior Pentagon official said Monday.
There were also fresh pledges of Western aid.
The Pentagon announced military aid for Ukraine worth $1 billion including additional precision ammunition for the Himars system that has helped Kyiv's forces attack Russian troops far behind the front lines.
The World Bank said it was mobilising another $4.5 billion in U.S.-provided financial support to help Kyiv pay for services and pensions, key to easing the invasion's economic impacts.
But the United Nations said Monday that more than 17 million people are in need of humanitarian assistance due to the Ukraine conflict, and increased an appeal for funds to respond to the crisis from $2.25 billion to $4.3 billion.
Fresh fine for journalist
The authorities in Moscow pursued their crackdown on opposition to the war, handing down a new fine to journalist Marina Ovsyannikova.
Ovsyannikova shot to prominence in March for interrupting a live broadcast to denounce Russia's military intervention in Ukraine.
On Monday, her lawyer did not rule out the possibility of her facing another criminal probe.
A Ukrainian court meanwhile jailed a Russian tank crewman for 10 years for firing on a multi-storied apartment block in the northern city of Chernihiv early on in the invasion, the security services (SBU) announced Monday.
The Chernihiv court convicted sergeant Mikhail Kulikov of a war crime.
The SBU also said it arrested Russian agents who were planning to assassinate the defense minister and the military intelligence chief.
There was progress on one rare area of compromise between Russian and Ukraine Monday, after the first cargo ship that left a Ukrainian port reached its final destination under a recent deal brokered by Ankara and the UN docked in Turkey.
While Kyiv welcomed the news, officials also said there was a delay in the first delivery to leave Ukrainian ports because the buyer had refused delivery, citing a five-month delay in shipment.
Infrastructure Minister Oleksandr Kubrakov said on social media that a cargo ship carrying grain had departed for the first time from the Black Sea port of Pivdennyi.