After almost two weeks of intense negotiations, experts from more than 70 countries gathered in Ljubljana adopted a document to cooperate in the investigation and prosecution of the crime of genocide, crimes against humanity and war crimes. It is the first such treaty since the 1998 Rome Statute.

"This will finally fill the legal vacuum in the field of international legal aid & extradition against alleged perpetrators of the most serious international crimes," the Slovenian Foreign Ministry tweeted as the biggest diplomatic conference ever hosted by Slovenia came to a successful close on 26 May.

Almost 300 international law and international criminal law experts were involved in finalising the document, called the Ljubljana-Hague Convention or the Convention on International Cooperation in the Investigation and Prosecution of Genocide, Crimes against Humanity, War Crimes and Other International Crimes.

Entailing nearly 100 articles, in addition to the preamble and final provisions, the convention determines central cooperation bodies and defines status of victims, witnesses, experts and other persons. It tackles international legal assistance and extradition procedures, transfer of convicted persons and dispute settlement procedures.

What is the first major international treaty on international criminal law since the Rome Statute is expected to be signed in The Hague early next year. It will need to be ratified by at least 15 countries to enter into force.

As the conference wrapped up, Justice Minister Dominika Švarc Pipan said the delegates floated the idea that the convention could come into force as soon as it was signed by two countries, "as this would enable first bilateral cooperation".

The document caps more than 12 years of efforts. The Netherlands, Belgium and Slovenia established the Mutual Legal Assistance and Extradition Initiative in 2011, and were later joined by Argentina, Senegal and Mongolia as the core members of the group.

"The adoption of the Ljubljana-Hague Convention is a historic achievement that represents a major step forward in our common fight against impunity and the search for justice for the most serious crimes. This Convention is turning the oft-stated words 'Never again!' into action," Foreign Minister Tanja Fajon said in her closing address.

She is confident that the convention will strengthen cooperation between countries in the investigation and prosecution of the most serious international crimes and enhance capacity to fight impunity.

"It will enable the international community as a whole to remain true to its commitment to the principle of accountability. It will remove existing legal loopholes and administrative obstacles and facilitate mutual legal assistance in criminal investigations, asset recovery, extradition and other international legal matters," she said.

She believes the adoption of the convention sends a clear message to all potential perpetrators and all future victims of these atrocious crimes that justice will prevail.