Jovica Stanisic, former head of Serbia's state security service, and his subordinate Franko "Frenki" Simatovic have each been sentenced to 12 years in jail for their roles in the 1990s Balkan wars in which 200,000 people died.
UN judges have convicted two men of war crimes for their role in financing and equipping Serb militias during the break-up of the former Yugoslavia, in the final case before the court dating from the Balkan wars of the 1990s.
In a summary of the judgment provided by the court, the court convicted the former head of Serbia's state security service Jovica Stanisic and his subordinate Franko "Frenki" Simatovic and handed them 12-year sentences.
They served as aides of the late Yugoslav president Slobodan Milosevic.
Stanisic and Simatovic were handed over to the court in 2003 and acquitted in 2013.
But appeals judges ordered a retrial in 2015.
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Maximum life sentence
Prosecutors had asked for the maximum life sentence for Stanisic and Simatovic, who are now in their early 70s. Both have pleaded "not guilty" to crimes including murder and persecution.
Prosecutors say the men helped to finance, train and equip Serb militias in Bosnia and Croatia that conducted brutal "ethnic cleansing" campaigns against non-Serbs, leaving thousands dead and driving 340,000 from their homes.
"These two men where critical in making sure the war was fought the way it was fought," said Iva Vukusic, a historian at Utrecht University specialising in former Yugoslav paramilitary units.
"They are the men behind the scenes, they are the ones responsible" for atrocities in Bosnia and Croatia, Munira Subasic, who lost her husband and son in the 1995 Srebrenica genocide, told Reuters outside the court building.
Bosnian Serb soldiers and paramilitaries killed 8,000 Muslim men and boys after they overran the UN-declared safe zone of Srebrenica.
Prosecutor Serge Brammertz told journalists last month that the case provided a "direct link to Belgrade and the Milosevic regime" from atrocities committed in Bosnia and Croatia.
Milosevic died in his cell in 2006 while on trial for genocide and crimes against humanity.