Russian forces are firing at a nuclear research facility in the city of Kharkiv, Ukrainian officials said.
The nation's parliament announced it was not possible to estimate the damage due to the ongoing clashes in the area.
It comes two weeks after a previous Russian attack on Kharkiv's Institute of Physics and Technology, which is home to a nuclear reactor core.
A Ukrainian politician warned that the latest shelling risked a 'major environmental disaster'.
The State Nuclear Regulatory Inspectorate of Ukraine said: "The continuation of its bombardment can lead to severe radiation consequences with contamination of nearby territories."
Emine Dzheppar, Ukraine's first deputy minister of foreign affairs, added: "The Russian aggressor fired rocket 'Grad' systems on the territory of the Kharkiv Institute, where the nuclear installation 'Source of Neutrons' is located, in the active zone of which 37 fuel nuclear cells were loaded.
"The destruction of a nuclear facility and nuclear material storage facilities could lead to a major environmental disaster. Ukraine continues to gather evidence of (Russian) war crimes to The Hague."
The regulator previously accused Russia of committing an "act of nuclear terrorism" by bombing the research facility.
In a statement the nuclear watchdog said: "Prior to the Russian aggression, the Source of Neutrons was at the stage of physical start-up, during which the core was loaded with fresh nuclear fuel.
"On February 24, 2022, the unit was transferred to a deep subcritical state by the operational personnel of the Source of Neutrons."
In a report on the state of the reactor, which is used in scientific research at the institute, the Inspectorate said critical buildings at the site had sustained damage during the attack but the reactor was in a "deep subcritical state" and "completely de-energized".
Video from the scene is understood to show huge flames at the centre, where the first development of a nuclear bomb was carried out in the USSR.
Ukraine's Centre for Strategic Communications and Information Security posted on Twitter: "In Kharkiv, eyewitnesses report a fire in the building of Kharkiv Institute of Physics and Technology.
"The building contains equipment which, if damaged, can lead to radioactive pollution of the environment."
After it was first targeted, authorities said the institute is home to a nuclear research facility with 37 nuclear fuel cells.
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Before Russia ’s invasion of Ukraine, while Vladimir Putin ’s troops gathered on the border, Kharkiv was seen as one of the cities most at threat and it has withstood horrific bombings and slaughter.
The resistance of the city has typified the response from Ukrainians since Russia’s invasion on February 24 and residents have refused to surrender.
Kharkiv Mayor Igor Terekhov said hundreds of buildings, many residential, had been destroyed.
"It is impossible to say that the worst days are behind us, we are constantly being bombed," he said.
A witness in the city said she saw people on the roofs of apartment buildings dropping grenades or similar ordnance onto the streets as the resistance continues.
