The United Nations has urged "maximum restraint" from Kyiv and Moscow following Ukraine's surprise invasion of Russia's Kursk region.

A statement was released by the head of the UN's nuclear watchdog agency amid fears that the fighting within Russia is getting increasingly close to the Kursk Nuclear Power Plant.

The plant has a total of six nuclear reactors, two of which are currently fully operational.

The UN's Rafael Grossi said: "At this juncture, I would like to appeal to all sides to exercise maximum restraint in order to avoid a nuclear accident with the potential for serious radiological consequences."

He added: "I am personally in contact with the relevant authorities of both countries and will continue to be seized of the matter."

Ukraine unexpectedly carried out its cross-border action earlier this week, with the Kremlin caught off-guard.

Troops have made about a six mile incursion of Russia, with thousands of locals forced to evacuate.

It comes as President Volodymyr Zelensky admitted for the first time last night that Ukraine had launched a surprise offensive into Russia.

In his nightly video address, Zelensky said he had discussed the operation with top Ukrainian commander Oleksandr Syrskyi and vowed to restore justice.

"Today, I received several reports from Commander-in-Chief Syrskyi regarding the front lines and our actions to push the war onto the aggressor's territory," he said.

"Ukraine is proving that it can indeed restore justice and ensure the necessary pressure on the aggressor."

Moscow's forces were in their sixth day of intense battles against Kyiv's largest incursion into Russian territory since the start of the war, which left southwestern parts of Russia vulnerable before reinforcement started arriving.

In a sign of the gravity of the situation, Russia imposed a sweeping security regime in three border regions on Saturday, while Belarus, a staunch ally of Moscow, sent more troops to its border with Ukraine, accusing Kyiv of violating its air space.

Russia's defence ministry said this morning it had destroyed 14 Ukrainian drones and four Tochka-U tactical ballistic missiles overnight over the Kursk region, and 18 drones over other Russian regions that Ukraine frequently attacks.

In a meeting with Russian government officials earlier this week, Putin described Ukraine's actions as a "major provocation".