A UN expert on illegal Libyan weapons, Dr Moncef Kartas, is being kept in prison by the Tunisian government in violation of international law, a letter signed by more than 90 weapons experts, academics and Libyan researchers has claimed.

Kartas was seized four weeks ago by Tunisian police and accused of espionage and contacts with foreign parties.

The letter, which is also signed by his wife and his brother, reflects a belief that Kartas, a German-Tunisian dual national, has been imprisoned simply for carrying out legitimate investigation, on behalf of the UN, of breaches of its Libyans arms embargo.

He is one of six members of the UN panel of experts investigating breaches of UN sanctions in Libya. The group produces an annual report for the UN security council that has in the past detailed wholesale breaches of the arms embargo, notably but not exclusively by the United Arab Emirates.

The UAE is an ally of Gen Khalifa Haftar, the eastern warlord who is besieging the Libyan capital, Tripoli. Tunisia has largely taken a neutral stance about the war on its borders.

In the letter, also published by Le Monde and Der Spiegel, the signatories say that Kartas enjoys immunity under international law as a UN employee and that at the time of his arrest he was carrying out his official duties. They also point out he could face the death penalty.

“We personally know and value Dr Kartas as a committed researcher and practitioner with impeccable ethical and professional standards,” the letter says.

It also calls on the UN secretary general and the German government to do more to demand his release. The UN has sent a series of private notes seeking his release, but is finding itself in a battle to retain its authority, including its ability to protect those who work on its behalf.

In its brief statement referring to the initial arrest, the Tunisian interior ministry said: “A number of secret documents containing accurate and sensitive data and data have been seized that would compromise the safety of national security, as well as technical equipment used in our country, which can be used for jamming and intercepting communications as used in radio surveys.”