Judges at the International Criminal Court (ICC) have added a year to Jean-Pierre Bemba's 18-year jail term following the former Congolese vice president's conviction for attempting to bribe witnesses during his war crimes trial.

After a second trial on the separate charges, Bemba on Wednesday was also ordered to pay a 300,000-euro ($323,670) fine to the court's fund supporting victims of atrocities.

Bemba was found guilty last year of bribery, and the verdict and sentence are the first of their kind in the history of the ICC.

"The chamber imposes on you an additional 12 months, one-year, imprisonment," presiding judge Bertram Schmitt told Bemba, adding a "substantial fine" was necessary "to discourage this kind of behaviour".

Several of the court's cases against prominent and powerful politicians have been weakened by witness bribery.

The decision to bring bribery charges against Bemba and four members of his legal team was seen by criminal law experts as a sign that prosecutors had decided to get tough on a practice that threatened their ability to prosecute the gravest crimes.

Bemba is appealing against his conviction last year on charges of having committed war crimes by allowing his forces to go on a murderous rampage through the neighbouring Central African Republic in 2002 and 2003.

His former lead lawyer, Aime Kilolo, was given a 30,000-euro fine and a two-and-a-half-year suspended sentence. Three others were given either suspended jail terms or sentenced to time served.

Judges gave prosecutors and the defence 30 days to lodge an appeal.