France was accused of an "unacceptable" attack on press freedom Tuesday after a journalist was arrested for reporting on leaked documents that claimed French intelligence was used to target civilians in Egypt.

Investigative website Disclose published a series of articles in November 2021 based on hundreds of secret documents.

It said they showed how information from a French counter-intelligence operation in Egypt, codenamed "Sirli", was used by the Egyptian state for "a campaign of arbitrary killings" against smugglers operating along the Libyan border.

On Tuesday, journalist Ariane Lavrilleux's home was searched and she was arrested for questioning by agents of the DGSI, France's domestic intelligence agency, Disclose announced on X (formerly Twitter).

It denounced an "unacceptable attack on the secrecy of sources" -- a view quickly backed by the Society of Journalists and Reporters Without Borders (RSF).

"We fear that the DGSI's actions will undermine the secrecy of the sources," RSF said.

Lavrilleux's arrest was confirmed to AFP by a source close to the case.

"I am appalled and worried about the escalation in attacks on the freedom to inform, and the coercive measures taken against the Disclose journalist," said Virginie Marquet, lawyer for Lavrilleux and Disclose.

"This search risks seriously undermining the confidentiality of journalists' sources," she said, adding that Lavrilleux had "only revealed information of public interest".

The initial Disclose articles said French forces were complicit in at least 19 bombings against smugglers between 2016 and 2018 in the region.

The documents showed there were warnings from officials within the French government, but the operation was not called into question, Disclose said.

France's Ministry of the Armed Forces filed a complaint for "violation of national defence secrecy" following the publication of the article, and a case was opened in July 2022 by the Paris prosecutor's office that was then placed in the hands of the DGSI.