Russian-backed separatists in eastern Ukraine reported on Sunday that the Ukrainian army had shelled the city of Donetsk.

According to Russian state news agency Tass, Mayor Alexey Kulemzin said that an administrative building had been heavily damaged, windows had been shattered and cars had caught fire.

"Miraculously, no one died," Kulemzin was quoted as saying. The information could not be independently verified. The industrial city of Donetsk has been under the control of pro-Russian separatists since 2014.

The Moscow-backed forces blamed the Ukrainian army for 40 attacks on targets in the Russia-recognized "Donetsk People's Republic" within 24 hours. One civilian was killed and four others were injured, they said.

Meanwhile, at the Zaporizhzhia nuclear power plant occupied by Russian troops, the situation is worsening with each passing week, according to the head of Ukraine's nuclear authority.

Russian occupiers have damaged the administration building, a training center and a part of the nuclear power plant, Petro Kotin told German public broadcaster ZDF.

"At the moment they are trying to pressure staff to sign work contracts with Russia," Kotin said. "The staff have the choice of either signing this contract or being beaten or tortured."

In shuttle diplomacy between Russia and Ukraine, International Atomic Energy Agency chief Rafael Grossi has been trying to get a safe zone established around the nuclear plant where there would be no fighting.