The landmark trial of wartime Bosnian Serb leader Radovan Karadzic will resume Monday at the UN's Yugoslav war crimes court in the Hague after a brief recess.

Karadzic has been charged with genocide, war crimes and crimes against humanity and the trial could last four years, the UN tribunal said Friday.

Judges warned the trial could stretch into 2014 - two years longer than expected - if prosecutors and the former Bosnian Serb leader do not speed up the case.

Karadzic faces 11 charges, including two counts of genocide, for allegedly masterminding atrocities throughout the Bosnian war. Judges have accused him of taking an excessive amount of time to question witnesses and using the court as a platform for political rhetoric.

Judge Burton Hall appealed to prosecutors and defence to be more efficient in presenting their evidence and in cross-examining witnesses.

"This is the biggest and most difficult process and it is expected to last long, but our goal is that it be efficient and just," Hall said.

Karadzic's lawyer Peter Robinson urged judges to scrub out charges relating to Serb ethnic cleansing of some 20 Bosnian municipalities early in Bosnia's 1992-95 war, which he said could cut a year off the trial.

So far, testimony has concentrated on the siege of the Bosnian capital, Sarajevo, where up to 12,000 people were killed by Bosnian Serb forces in the 44-month siege of Sarajevo.