Liberian ex-president Charles Taylor are expected to have a decision on him by September while he will remain behind bars at the UN's detention unit at The Hague until appeals proceedings are finalized.
If his appeal fails, Mr. Taylor will serve his sentence in a British jail.
Taylor began his appeal against a 50-year prison sentence on Tuesday with his lawyers arguing he could not be linked to weapons used by rebels to commit brutal crimes during the African nation's savage civil war.
"There is nothing... to indicate that Charles Taylor knew that specific weapons or ammunition he may have had some role in providing would be used in a crime," defence lawyer Christopher Gosnell told judges before Sierra Leone's UN-backed special court in The Hague.
Gosnell said that unlike during the 1994 genocide in Rwanda in which many of the victims were hacked to death as part of ethnic cleansing, the arms and ammunition supplied by Taylor were to be used as part of a legitimate war.
"It's not a case of a million machetes being shipped to Rwanda. It was ammunition used in a military campaign," Gosnell told the Special Court for Sierra Leone.
Taylor, wearing a black suit, white shirt and red tie, listened intently as the prosecution and defence engaged in a complex game of legal ping-pong at the court's headquarters in Leidschendam outside The Hague.
Opening the prosecution's case earlier, Nicholas Koumjian told a six-judge appeals bench the court should "hold responsible not only those who perpetrate the crimes, but also those who promote them."
Fomentors of war like Taylor "are just as important. Those are the promoters of war, the lords of war who sell arms in these conflicts," Koumjian said.
The court's sentence last May against Taylor, 64, for "some of the most heinous crimes in human history", was widely welcomed around the world at the time.
Judges said he aided and abetted rebel forces fighting against Freetown during Sierra Leone's 10-year civil war, known for its mutilations, drugged child soldiers and sex slaves.
In return, trial judges found, Taylor was paid in "blood diamonds" mined by slave labour in areas kept under the control of ruthless Sierra Leonean rebels.
But prosecutors argue that trial judges made a mistake by only convicting Taylor of aiding and abetting the notorious Revolutionary United Front and other rebel groups.
They say the court should have convicted Taylor for actively issuing orders to the RUF and its ally, the Armed Forces Revolutionary Council (AFRC).
Taylor was convicted of aiding and abetting terrorism, murder and rape, committed by the RUF, who waged a terror campaign during a civil war that claimed 120,000 lives between 1991 and 2001.
The initial trial, which saw model Naomi Campbell testify she had received a gift of "dirty" diamonds, said to be from the flamboyant Taylor, wrapped up in March 2011.
His sentence was the first handed down against a former head of state in an international court since the Nazi trials at Nuremberg in 1946.
Koumjian said that as a result, the trial and the appeals verdict is "of great consequence".
Tuesday's hearing was dominated by complex legal arguments -- with both sides saying judges made legal mistakes in convicting and sentencing Taylor.
"A final sentence must reflect the totality of the crimes," SCSL chief prosecutor Brenda Hollis -- who is asking for Taylor to be jailed for 80 years -- told judges.
Prosecutor Koumjian hit back at the defence.
"In their view as long as their purpose is not crimes, but advantages, in the case of Charles Taylor the diamonds of Sierra Leone, then it's OK, they're not responsible for aiding and abetting," he said.
Should the appeals judges find the defence's argument valid, it "would be a great step backwards in international law," he added.
Taylor's defence has filed 42 grounds of appeal, calling the trial chamber's decision a "miscarriage of justice", and asked appeals judges to reverse the conviction and quash the sentence.