Former Bosnian Serb leader Radovan Karadzic being tried here for war crimes said he was "sorry" for the arrest of ex-army chief Ratko Mladic in Serbia Thursday, his lawyer Peter Robinson said.

"I was at the prison with president Karadzic this afternoon. President Karadzic is sorry for General Mladic's loss of freedom," Robinson said in an email to AFP.

"He looks forward to working with him to bring out the truth about what happened in Bosnia," Robinson added.

Mladic was arrested in Serbia earlier Thursday and like Karadzic faces charges of genocide, war crimes and crimes against humanity before the International Criminal Tribunal for the former Yugoslavia (ICTY) in The Hague.

The trial of Karadzic, arrested in Belgrade in 2008 after 13 years on the run, opened in October 2009.

Like Mladic, he has been arrested for his role in the 1992-1995 Bosnian war, which saw 100,000 people killed and 2.2 million others displaced.

Karadzic is being held at the ICTY's penal facility in Scheveningen, where he was expected to be joined by Mladic.