The Working Group on Arbitrary Detention is a little-known piece of a sprawling human rights organization. Neither a court nor a tribunal, the five-member body can call countries and their leaders to account, but has no power of enforcement.

Here's a look at the group, its mandate, and the challenges it faces.

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What is it?

The working group is composed of five experts selected by the leadership of the U.N.'s Human Rights Council for three-year terms, renewable only once. Members are unpaid. The group is among 55 "special procedures" authorized under the council, which are mostly special rapporteurs on issues ranging from the rights of albinos to those of indigenous peoples.

The panel was set up in 1991 by the former Commission on Human Rights "to investigate instances of alleged arbitrary deprivation of liberty," according to U.N. human rights office. Panel member Roland Adjovi said it meets three times a year in Geneva for a total of no more than 20 days — limited because of U.N. budgetary reasons.

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Who are its members?

The chair is international law professor Seong-Phil Hong of Korea; Mexico's Jose Guevara is a human rights specialist at Carlos III University in Madrid and Setondji Roland Adjovi of Benin is an assistant professor of African affairs and international law at Arcadia University near Philadelphia in the United States.

Vladimir Tochilovsky, an expert in international criminal justice who has worked with the International Criminal Court, was the lone dissenting voice. Human rights expert Leigh Toomey, who was appointed last year, recused herself because she, like Assange, is Australian.

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What is its caseload?

Anyone who claims to be deprived of liberty can submit a case to the panel, and its workload has been growing. Since it was founded, the working group has produced more than 1,000 opinions involving more than 130 countries. Last year, it delivered 56 opinions — one fewer than in 2014.

Christophe Pechoux, a section chief for special procedures of the Human Rights Council, said at least 12 people were freed from arbitrary detention last year, but it could be more because sometimes those who are freed don't inform the panel.

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Are its decisions binding, and if so, how are they enforced?

That's a matter of some dispute. Britain's government and Swedish prosecutors insist the panel's decision changes nothing. But the U.N. body counters that the two countries have ratified the International Convention on Civil and Political Rights, and as such are bound to apply the opinion of the panel, and international conflict-resolution and dispute-resolution mechanisms.

"When you combine all these elements, you have a binding decision," said Peschoux. "Who is going to enforce it? Nobody."