The United Nations mission in Libya on Saturday called for the "immediate" release of a prominent journalist arrested this week, warning against a "crackdown" on media freedoms in the war-torn country.

Ahmed Sanussi, chief editor of Libyan financial news website Sada who has long covered corruption in the hydrocarbon-rich country, was arrested in his Tripoli home after returning from Tunisia, his family said.

The UN Support Mission in Libya (UNSMIL) said it was "deeply concerned about the arbitrary arrest and detention of journalist Ahmed Sanussi on July 11 in Tripoli".

In a message on social media platform X, UNSMIL called for his "immediate release".

"The crackdown on journalism fosters a climate of fear and undermines the necessary environment for democratic transition in Libya," it said.

Libya has been wracked by division and unrest since the 2011 NATO-backed overthrow of former dictator Moamer Kadhafi, and remains divided between two rival administrations.

The UN mission highlighted the need for a "thriving civic space where Libyans can engage in open and safe debate and dialogue by exercising their right to freedom of expression".

"All Libyan authorities must protect journalists and media professionals."

Sanussi's latest reporting on corruption implicated Economy Ministry Mohamad Ali Houej.

Authorities in Libya did not comment on the arrest, which was also condemned by Western governments.

The Committee to Protect Journalists (CPJ) deemed it "unacceptable that authorities have not disclosed where he is being held or the reason for his arrest".

The Netherlands' ambassador in Libya, Joost Klarenbeek, said on X he was "deeply concerned", adding that "any acts of arbitrary detention, enforced disappearance or ill-treatment must be thoroughly investigated".

CPJ's MENA program coordinator, Yeganeh Rezaian, said Libyan "authorities must release Sanussi immediately and unconditionally and ensure his safe return home".