Former Bosnian Serb leader Radovan Karadzic has filed an appeal against a UN court's decision to increase his sentence to life in jail for war crimes, according to court documents.

Judges in The Hague will now have to decide whether to admit Karadzic's challenge to his sentence, as the 20 March ruling on his sentence was supposed to be final.

"President Radovan Karadzic hereby appeals...  the life sentence imposed upon him", said the legal notice filed by his lawyer Peter Robinson and released by the court.

Appeals judges in The Hague last week upheld Karadzic's 2016 conviction for genocide in the Srebrenica massacre and other atrocities throughout the Bosnian civil war in the 1990s.

They then increased his sentence from 40 years to life, saying the initial jail term had underestimated the "sheer scale and systematic cruelty" of his crimes.

Karadzic's appeal cited eight grounds to challenge the 20 March ruling despite the fact that decisions by the appeals chamber of the UN tribunal are usually decisive.

"A prospective appeal raises complex issues, including whether an appeal of an impugned decision is even receivable by the appeals chamber," the legal filing said.

But it said that while in previous cases the court found a person cannot challenge a conviction if it is upheld on appeal, "President Karadzic's appeal may be distinguishable since he seeks to appeal only his sentence." 

The 73-year-old also sought legal aid to pay for lawyers to bring the appeal, the court said.