Facebook has reported BBC journalists to the police after they provided examples -- at the company's request -- of obscene images of children posted on its platform.

The highly unusual episode was reported Tuesday by the BBC as part of a larger investigation into how Facebook handles images of child exploitation on its social network.

The BBC says it requested an interview with a Facebook executive after finding that the company had removed only 18 of 100 images its journalists had flagged as obscene via the social network's own "report button."

Facebook (FB) agreed to do an interview, but only if the BBC would provide examples of the material, which included Facebook pages explicitly for men with a sexual interest in children and Facebook groups with names like "hot xxxx schoolgirls."

One particularly disturbing image appeared to be taken from a video of child abuse, the BBC said.

When the BBC complied with Facebook's request to send the material, the social network responded by canceling the interview and reporting the network's journalists to the U.K.'s National Crime Agency.

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Facebook policy director Simon Milner defended the company's actions on Tuesday, saying in a statement that it's "against the law for anyone to distribute images of child exploitation."

"When the BBC sent us such images we followed our industry's standard practice and reported them," he said. "We also reported the child exploitation images that had been shared on our own platform."

Facebook said it has now removed all items that were illegal or against its standards.

"We take this matter extremely seriously and we continue to improve our reporting and take-down measures," Milner said.

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Damian Collins, a U.K. lawmaker who chairs a parliamentary media committee, told CNNMoney that he was troubled by the fact that Facebook allowed content that included images of child abuse to remain on the site even after they were flagged as inappropriate.

He also described the firm's interactions with the BBC as "astonishing."

"They should take a bit more responsibility and someone from the company should put themselves forward for an interview," he said. "It can't be the case that Facebook's only job is to refer that content to the police."

A spokesman for the U.K.'s National Crime Agency said it does not comment on specific cases.

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Facebook has run into trouble over issues related to censorship, free speech and objectionable content in the past.

Its policies prohibit nudity, hate speech or graphic images that glorify violence. It has also banned the private sale of guns and drugs.

Other areas are less clear. Facebook, for example, reversed course last year and allowed users to post the iconic "Napalm Girl" image after facing fierce criticism for censoring one of the most famous war photographs in history.

"It's a question of the responsibility," Collins said of objectionable content. "My feeling is they need to put a lot more resources into this area."