LVIV, Ukraine, Aug 18 (Reuters) - U.N. Secretary-General Antonio Guterres called on Thursday for the demilitarisation of the vast nuclear power plant held by Russia in southern Ukraine's Zaporizhzhia region, and said he was gravely concerned by the situation in and around it.

Guterres, speaking to reporters after talks in the western Ukrainian city of Lviv, said that military equipment and personnel should be withdrawn from the plant and called for efforts to ensure it is not the target of military operations.

"The facility must not be used as part of any military operation. Instead, agreement is urgently needed to reestablish Zaporizhzhia's purely civilian infrastructure and to ensure the safety of the area," he said.

(Reporting by Andrea Shalal; writing by Tom Balmforth; editing by Timothy Heritage)