A former Bosnian Serb general and close aide to top war crimes suspect Ratko Mladic has been arrested in Bosnia.
Tolimir is among the top fugitives from the UN war crimes court
Zdravko Tolimir, 58, is one of the top fugitives sought by the UN war crimes tribunal in The Hague for his alleged role in the Srebrenica massacre.
Gen Mladic has evaded arrest for years. He is wanted for war crimes against Muslim civilians in the 1990s.
Correspondents say Mr Tolimir's arrest, confirmed by the Hague tribunal, could speed up the search for his ex-boss.
He was detained in eastern Bosnia close to the border with Serbia and just a few kilometres from the town of Srebrenica, where up to 8,000 Bosnian Muslim men and boys were killed in July 1995.
Preparations are being made for his transfer to The Hague.
A Bosnian Serb spokeswoman said the arrest came as a result of a joint operation with Serbia.
Rasim Ljajic, head of Serbia's office for cooperation with the International Criminal Tribunal for the Former Yugoslavia (ICTY), said the detention showed a new determination on the part of Belgrade.
"This is the first time that not a single fugitive from the ICTY can sleep peacefully, because our civilian and military intelligence will work simultaneously on locating them," he said.
The EU has linked the resumption of talks on closer ties with Serbia to the arrest of war crimes suspects.
Olga Kavran, spokeswoman for the chief UN war crimes prosecutor for former Yugoslavia, Carla Del Ponte, said they were informed of the arrest by the Bosnian Serb Prime Minister Milorad Dodik.
"We understand [he] has some health issues and we will try to assess what these issues are in order to make arrangements for his transportation to The Hague," a tribunal spokesman told Reuters.
The BBC's Nicholas Walton in Sarajevo says the arrest of Gen Tolimir is an important breakthrough for those hunting other fugitives wanted in connection with the Bosnian War of the early 1990s.
Radovan Karadzic and Gen Mladic are still on the run
He was considered the third most important person on the list of those indicted for war crimes by the ICTY, after Gen Mladic and his wartime political leader, Radovan Karadzic.
Both Gen Mladic and Mr Karadzic have been indicted for genocide over the killings in Srebrenica and the 43-month siege of Sarajevo, which claimed more than 10,000 lives.
Gen Tolimir was an intelligence officer and senior aide to Gen Mladic at the time of the massacre at Srebrenica, which was carried out by Bosnian Serb soldiers under Gen Mladic's command.
Gen Tolimir is accused of helping to plan and carry out the murders, which have since been internationally recognised as genocide.
With his arrest, only five of the 161 people indicted by the UN tribunal remain at large, including Gen Mladic and Mr Karadzic.
Gen Tolimir is thought to have been one of the key figures helping his former commander to evade capture.
If so, he may provide important information about where the most wanted war crimes suspect is now hiding, our correspondent says.