A former Bosnian Serb intelligence chief arrested last week has accused UN prosecutors in The Hague of breaking their own rules in arresting him.
Gen Tolimir is a suspect in the 1995 Srebrenica massacre
In his first court hearing, Gen Zdravko Tolimir said he had been sent against his will to the Bosnian Serb republic, even though he is a citizen of Serbia.
UN war crimes tribunal chief prosecutor Carla Del Ponte is visiting Serbia to assess its co-operation with the court.
The EU has pledged to resume talks soon on closer ties with Serbia.
Tolimir, 58, did not enter a plea to charges of genocide, extermination, persecution and inhumane acts.
He told the court he had suffered three strokes and lost around 30kg, and said that he was unlawfully extradited to the Bosnian Serb republic (Republika Srpska).
Carla Del Ponte is dealing with a new Serbian government
"I was arrested in the territory of Serbia, whose citizen I am, and I was transferred to the territory of [Republika Srpska] ... without the presence of a lawyer," Gen Tolimir said.
He was reportedly arrested following a tip-off from Serbian police after spending years on the run.
He was a top aide to Gen Ratko Mladic, the most-wanted former Bosnian Serb commander, who is still on the run.
His arrest reduced to five the number of indicted war crimes suspects still at large.
He is accused of involvement in the Srebrenica massacre of up to 8,000 Muslim men and boys in eastern Bosnia in 1995.
Ms Del Ponte's visit was requested by the new Serbian government, which wants her to assess its co-operation with the court.
She has repeatedly urged Belgrade to capture Gen Mladic.
The European Union suspended talks with Serbia a year ago because of Belgrade's failure to co-operate with the court.
The resumption date will depend on Ms Del Ponte's assessment.
EU Enlargement Commissioner Olli Rehn has said he expects the talks to resume later this month.