Russia's seizure of the plant "poses a serious threat to the safety and the security of this facility (and) raises the risks of a nuclear accident or incident", he told reporters in Brussels.

"It is urgent to allow the inspection by the International Atomic Energy Agency and to ensure the withdrawal of all Russian forces," he said.

The Russian military control of the facility "endangers the population of Ukraine, of neighbouring countries and of the international community", Stoltenberg added.

"Russian troops... now use the ground around the nuclear power plant as a staging area, as a platform, to launch artillery attacks on Ukrainian forces, and this is reckless, it is irresponsible."

Stoltenberg made the comments in separate news conferences Wednesday with the presidents of Serbia and Kosovo.

Russian forces in March took the Zaporizhzhia nuclear power plant, located in southern Ukraine, shortly after invading.

The plant is the largest one in Europe, and the uncertainty surrounding it as the war rages has fuelled fears of a nuclear accident to rival that of Chernobyl in 1986, when a reactor exploded.

Russia and Ukraine have accused each other of shelling the Zaporizhzhia nuclear installation.

The UN Security Council last week held an emergency meeting over the situation and warned of a "grave" crisis unfolding in Zaporizhzhia.

On Thursday, UN Secretary General Antonio Guterres will hold talks in western Ukraine with Ukrainian leader Volodymyr Zelensky and Turkish President Recep Tayyip Erdogan. The discussions were expected to include Zaporizhzhia.

Ukraine counted on four nuclear power stations to supply it with around half of its electricity supply before Russia's invasion.

Its call for a demilitarised zone around the Zaporizhzhia facility has been backed by Western allies.