UPDATE 1--U.N. chief: Ukraine's nuclear plant must be demilitarised

(Adds background, detail about fact-finding mission)

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to reporters after talks in the western Ukrainian city of Lviv, said that military equipment and personnel should be withdrawn from the plant and called for efforts to ensure it is not the target of military operations.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

Russia and Ukraine have blamed each other for shelling the nuclear plant and on Thursday accused each other of preparing to stage a "provocation" at the vast complex.

The facility on the Russian-controlled south bank of a huge reservoir was captured by Russia in March.

"We must spare no effort to ensure that plant's facilities or surroundings are not a target of military operations," Guterres told reporters after talks with Ukrainian President Volodymyr Zelenskiy and Turkish President Tayyip Erdogan.

Earlier this month, Guterres launched a fact-finding mission into an incident in the front-line Ukrainian town of Olenivka in which prisoners held by Moscow-backed separatists were killed. Russia and Ukraine both requested an investigation.

Guterres said on Thursday that the terms for the mission and the make-up of the team had been shared with Russia and Ukraine, who both need to agree to it. He said he intends to appoint Brazilian General Carlos dos Santos Cruz to lead the inquiry.

"We will now continue to work to obtain the necessary assurances to guarantee secure access to the site and any other relevant locations," Guterres said. "To put it simply, a fact-finding mission must be free to find the facts." (Reporting by Andrea Shalal in Lviv and Michelle Nichols in New York; writing by Tom Balmforth; editing by Timothy Heritage)