The International Atomic Energy Agency said an inspection last week of the steel confinement structure completed in 2019 found the drone impact in February, three years into Russia's conflict in Ukraine, had degraded the structure.
IAEA director general Rafael Grossi said in a statement the inspection "mission confirmed that the (protective structure) had lost its primary safety functions, including the confinement capability, but also found that there was no permanent damage to its load-bearing structures or monitoring systems."
Grossi said repairs had already been carried out "but comprehensive restoration remains essential to prevent further degradation and ensure long-term nuclear safety".
The UN reported on February 14 that Ukrainian authorities said a drone with a high explosive warhead struck the plant, caused a fire and damaged the protective cladding around reactor Number Four, which was destroyed in the 1986 disaster.
Ukrainian authorities said the drone was Russian. Moscow denied it had attacked the plant.
Radiation levels remained normal and stable and there was no reports of radiation leaks, the UN said in February.
The 1986 Chernobyl explosion sent radiation across Europe and prompted Soviet authorities to mobilise vast numbers of men and equipment to deal with the accident. The plant's last working reactor was closed in 2000.
Russia occupied the plant and the surrounding area for more than a month in the first weeks of its February 2022 invasion of Ukraine as its forces initially tried to advance on the Ukrainian capital Kyiv.
The IAEA had conducted the inspection at the same time as a country-wide survey of damage to electricity substations by the nearly four-year war between Ukraine and Russia.
Separately, Ukraine's Zaporizhzhia nuclear power plant temporarily lost all off-site power overnight, the International Atomic Energy Agency said on Saturday citing Grossi.
The plant was reconnected to a 330-kilovolt (kV) power line after an half-hour outage, but a 750 kV line is still disconnected, the IAEA said.
Widespread military activities overnight affected Ukraine's electricity grid and prompted operating nuclear power plants (NPPs) to reduce output, the IAEA added.
