Serb ultranationalist Vojislav Seselj made false claims of witness intimidation during his ongoing war crimes trial, the International Criminal Tribunal for the former Yugoslavia (ICTY) said Wednesday.

An independent inquiry found that Seselj's claims against the prosecutor's office (OTP) were "unfounded", said the ICTY.

"Moving to the contempt of court allegations of intimidating and bribing witnesses that Seselj raised against the OTP (...) the amicus curiae (independent expert) concluded that Seselj's allegations were unfounded," said tribunal clerk Martin Petrov.

Seselj claimed that the ICTY had forced witnesses to testify against him, prompting the Hague-based tribunal to order an independent inquiry on June 29, 2010.

"The amicus curiae found that some of the statements obtained by Seselj were exaggerated whilst others were demonstrably false," said Petrov.

Seselj, 57, went on trial in November 2006 for his alleged role in the persecution of Croats, Muslims and other non-Serbs and their expulsion from areas of Croatia, Bosnia and Serbia between 1991 and 1994.

He faces nine counts of war crimes and crimes against humanity including persecution, murder, torture and cruel treatment.

On Monday UN judges handed Seselj a 18-month sentence for contempt of court, his second such sentence in two years, after finding him guilty of revealing details about protected witnesses in his trial in a book.

This sentence will run concurrently with a 15-month sentence given to him in July 2009 for disclosing the identities of three witnesses in his trial as well as portions of a confidential statement in another book.

He pleaded not guilty in July to a third contempt of court charge for not removing information published on his website, including three books he wrote and five confidential files which he submitted during his main trial.