The UN atomic watchdog has said it saw "no critical impact on safety" from the loss of power at Ukraine's Chernobyl nuclear power plant, site of the 1986 disaster.

"Ukraine has informed IAEA of power loss," the International Atomic Energy Agency (IAEA) said in a tweet, adding that "in this case IAEA sees no critical impact on safety".

Ukraine's energy operator Ukrenergo said power has been entirely cut to the plant, while Ukraine's state-run nuclear company Energoatom said fighting between Ukrainian and Russian forces made it impossible to immediately repair the high-voltage power line to the plant, which has been captured by Russian forces.

Ukraine warned that there could be a radiation leak if the electricity outage continued and officials have appealed to Russia for a temporary ceasefire to allow repairs to be made.

Energoatom said radioactive substances could be released if the plant cannot cool spent nuclear fuel.

The Foreign Minister Dmytro Kuleba said reserve diesel generators can power the plant for only 48 hours.

"After that, cooling systems of the storage facility for spent nuclear fuel will stop, making radiation leaks imminent," Mr Kuleba said on Twitter.

"I call on the international community to urgently demand Russia to cease fire and allow repair units to restore power supply."

The Umited Nations nuclear watchdog said the loss of power does not have a critical impact on safety.

But Energoatom said there were about 20,000 spent fuel assemblies at Chernobyl that could not be kept cool during a power outage, and that their warming could lead to the release of radioactive substances into the environment.

"The radioactive cloud could be carried by wind to other regions of Ukraine, Belarus, Russia, and Europe," it said in a statement.

The IAEA had earlier stated that the plant was no longer transmitting data.

International Atomic Energy Agency chief Rafael Grossi "indicated that remote data transmission from safeguards monitoring systems installed at the Chernobyl NPP had been lost", the agency said in a statement.

"The agency is looking into the status of safeguards monitoring systems in other locations in Ukraine and will provide further information soon," it said.

The IAEA uses the term "safeguards" to describe technical measures it applies to nuclear material and activities, with the objective of deterring the spread of nuclear weapons through early detection of the misuse of such material.

More than 200 technical staff and guards remain trapped at the site, working 13 days straight since the Russian takeover.

The situation for the staff "was worsening" at the site, the IAEA said, citing the Ukrainian nuclear regulator.

The defunct plant sits inside an exclusion zone that houses decommissioned reactors as well as radioactive waste facilities.

More than 2,000 staff still work at the plant as it requires constant management to prevent another nuclear disaster.

The UN agency called on Russia to allow workers to rotate because rest and regular shifts were crucial to the site's safety.

"I'm deeply concerned about the difficult and stressful situation facing staff at the Chernobyl nuclear power plant and the potential risks this entails for nuclear safety," said Mr Grossi.

"I call on the forces in effective control of the site to urgently facilitate the safe rotation of personnel there."

With remote data transmission cut off and the Ukrainian regulator only able to contact the plant by email, Mr Grossi reiterated his offer to travel to the site or elsewhere to secure "the commitment to the safety and security" of Ukraine's power plants from all parties.

Russia also attacked and seized Europe's largest atomic power plant, Zaporizhzhia, last week, drawing accusations of "nuclear terror" from Kyiv.

Zaporizhzhia alone has six reactors of a more modern, safer design than the one that melted down at Chernobyl.

The IAEA said two of those were still operating, the plant's personnel were working in shifts and radiation levels remained stable.