THE HAGUE, September 11, 2008 (AFP) - The UN's Yugoslav war crimes tribunal on Thursday jailed an unwilling witness in the war crimes trial of Serbian ultra-nationalist Vojislav Seselj.

Ljubisa Petkovic was found guilty of contempt of the International Criminal Tribunal for the Former Yugoslavia, and sentenced to four months' imprisonment.

Petkovic has already spent three-and-a-half months in custody awaiting trial and this would be taken off.

'Petkovic refused to comply with a confidential subpoena of 7 April 2008 which ordered him to appear as a ... witness in the case of Vojislav Seselj on 13 May 2008,' said a tribunal statement.

Petkovic had indicated he would not appear before the tribunal 'with any status other than a witness for the defence', it added.

'Witnesses are not the property of the parties and when the trial chamber decides, by way of subpoena, that their testimony is necessary for the establishment of the truth, they have to comply with it,' read the judgment.

Head of the Serb Radical Party, the biggest in the Serbian parliament, Seselj's trial was adjourned in August pending a decision by the court over a prosecution bid to strip him of the right to present his own defence.

His second trial for war crimes and crimes against humanity during the 1990s Balkan wars began in November 2007. A first trial was nullified when Seselj went on a hunger strike in 2006.

Seselj denies having joined former Serbian president Slobodan Milosevic to 'ethnically cleanse' large parts of Bosnia, Croatia and Serbia.

His trial is expected to run for another year.

According to the Serbian news agency Beta, Petkovic was chief of paramilitary headquarters of the Serbian Radical Party in 1991 and 1992.

The court is believed to have wanted to hear more about his role in recruiting volunteers and sending them to the wars in Croatia and Bosnia.