(CNN) -- An international court in the Netherlands ruled Thursday to uphold the 50-year sentence handed down last year to Liberia's former president, Charles Taylor, after he was convicted of aiding war crimes in neighboring Sierra Leone.

The ruling by the appeals judges in the United Nations-backed Special Court for Sierra Leone in The Hague brings to an end a lengthy legal battle.

Taylor, 65, was found guilty last year of supplying and encouraging rebels in Sierra Leone in a campaign of terror, involving murder, rape, sexual slavery, looting and the conscription of children younger than 15.

He was also convicted of using Sierra Leone's diamond deposits to help fuel its civil war with arms and guns while enriching himself with what have commonly come to be known as "blood diamonds."

Both the defense and prosecution lodged appeals after the court convicted the former president of all 11 counts against him, following a trial lasting nearly four years.

Taylor's defense appealed the court's judgment and sentence on multiple grounds, arguing that the trial chamber had made mistakes in evaluating the evidence and in applying the law.

But the appeals judges rejected those arguments, saying that the trial chamber had "thoroughly evaluated the evidence for its credibility and reliability," and that its assessment of Taylor's criminal responsibility and liability was in accordance with international laws.

The appeals judges also dismissed defense claims that Taylor was not given a fair trial.

The defense also argued that the 50-year sentence handed down was "manifestly unreasonable," while the prosecution had argued that it should be increased to 80 years to adequately reflect the gravity of his crimes.

The appeals judges dismissed both claims, saying the sentence was fair and reasonable.

Rights group Amnesty International welcomed the ruling, saying it sent a clear message to leaders around the world.

"The Court's landmark ruling underlines that no one is above the law," said Stephanie Barbour, head of Amnesty International's Centre for International Justice in The Hague.

"The conviction of those responsible for crimes committed during Sierra Leone's conflict has brought some measure of justice for the tens of thousands of victims. The conviction of Charles Taylor must pave the way for further prosecutions."

Role in atrocities

Taylor was the first former head of state to be convicted of war crimes since the Nuremberg trials that followed World War II.

The trial chamber heard that rebels from the Revolutionary United Front, which the former president backed, committed horrendous crimes against Sierra Leone civilians, including children. Some were enslaved to mine the diamonds used to fund the rebels' fight.

The presiding trial judge described Taylor as responsible for "aiding and abetting as well as planning some of the most heinous and brutal crimes recorded in human history."

But Taylor said during his sentencing hearing in May 2012 that his role in the conflict was much different than represented. "I pushed the peace process hard, contrary to how I have been portrayed in this court," he said.

A pivotal figure in Liberian politics for decades, he became president in 1997 and was forced out of office under international pressure in 2003. He fled to Nigeria, where border guards arrested him three years later as he was attempting to cross into Chad.

The United Nations and the Sierra Leone government jointly set up the special tribunal to try those who played the biggest role in the atrocities. The court was moved to the Netherlands from Sierra Leone, where emotions about the civil war still run high.

CNN's David McKenzie contributed to this report.