Facebook Inc will learn from a mistake it made by deleting a historic photo of a naked girl fleeing a napalm attack taken during the American war in Vietnam, the company's chief operating officer said.

The photograph was removed from several accounts on Friday, including that of Norwegian Prime Minister Erna Solberg, on the grounds it violated Facebook's restrictions on nudity. It was reinstated after Solberg accused Facebook of censorship and of editing history.

"These are difficult decisions and we don't always get it right," Facebook COO Sheryl Sandberg wrote in a letter to the prime minister, obtained by Reuters on Monday under Norway's freedom of information rules.

"Even with clear standards, screening millions of posts on a case-by-case basis every week is challenging," Sandberg wrote.

"Nonetheless, we intend to do better. We are committed to listening to our community and evolving. Thank you for helping us get this right," she wrote. She said the letter was a sign of "how seriously we take this matter and how we are handling it".

The 1972 photograph, by Pulitzer Prize-winner Nick Ut of the Associated Press, shows screaming children running from a napalm attack. A naked nine-year-old girl, Phan Thị Kim Phúc, is at its center.

Sandberg wrote that "sometimes ... the global and historical importance of a photo like 'Terror of War' outweighs the importance of keeping nudity off Facebook."

Facebook bars nudity with some exemptions, such as photographs of nudes in art. It is unclear exactly how disputes over its "Community Standards" reach top management.

Solberg posted the photograph on her Facebook page after the company had deleted it from the sites of Norwegian authors and the newspaper Aftenposten, which mounted a front-page campaign on Friday urging Facebook to permit publication.

Solberg welcomed Facebook's about-turn. "It shows that it helps to use your voice to say 'we want a change'. I'm very pleased with that," she told NRK public broadcasting on Friday.

Sandberg suggested that Solberg's staff could meet two Facebook officials visiting Norway on Friday. "I hope to see you soon - and am always available if you have further concerns," she wrote.

Norway is a big investor in Facebook. Its $891 billion sovereign wealth fund, the world's biggest, had a stake of 0.52 percent in Facebook, worth $1.54 billion at the start of 2016.

($1 = 8.2667 Norwegian crowns)