(AFP) - International Criminal Court judges on Monday convicted a Congolese rebel chief nicknamed "Terminator" of war crimes including horrific massacres of civilians, sexual enslavement and recruitment of child soldiers.

Former warlord Bosco Ntaganda becomes the first person to be convicted of sexual slavery by the tribunal in The Hague, in a badly-needed boost for prosecutors after a string of high-profile failures.

Ntaganda stood expressionless as judges passed sentence on a man they called a "key leader" in atrocities in eastern DR Congo's Ituri province in 2002-3, issuing "direct orders to target and kill civilians".

In one of their most gruesome attacks, his men disembowelled babies and children or crushed their skulls.

"Mr Ntaganda rallied the troops prior to battle, he gave direct orders to the troops during operations, and he debriefed them afterwards," head judge Robert Fremr declared.

Ntaganda was found guilty of 18 counts of war crimes and crimes against humanity, including murder, directing attacks against civilians, displacing civilians, rape, sexual slavery and enlisting children under the age of 15.

Ntaganda, 45, who was born in Rwanda, will be sentenced after the court hears submissions from victims. Judges can give a life sentence.

He is only the fourth person to be convicted of war crimes by the ICC. The tribunal was set up in 2002 to try the world's worst crimes but has been criticised for a series of prosecution failures and for focusing especially on African suspects.

- Bodies 'disembowelled' -

Rights groups said Ntaganda's conviction would send a signal to other war crimes suspects.

"The long-awaited judgment provides an important measure of justice for Bosco Ntaganda’s victims and puts others responsible for grave crimes on notice," said Maria Elena Vignoli of Human Rights Watch (HRW).

"But renewed violence in eastern Congo highlights the need to address the impunity for other abusive leaders."

Ntaganda -- known for his pencil moustache and a penchant for fine dining -- proclaimed his innocence throughout the trial, insisting that he was a "soldier not a criminal" and that the "Terminator" nickname did not apply to him.

Prosecutors portrayed him as the ruthless leader of ethnic Tutsi revolts amid the civil wars that wracked the DRC after the 1994 genocide of Tutsis in neighbouring Rwanda.

Judges said Ntaganda "fulfilled a very important military function" as a leader of the Union of Congolese Patriots rebels and its military wing, the Patriotic Forces for the Liberation of Congo (FPLC).

The FPLC killed at least 800 people as it fought rival militias in Ituri for control of valuable minerals. More than 60,000 people have been killed since violence erupted there in 1999.

In one attack directed by Ntaganda, judges said that soldiers killed at least 49 captives in a banana field behind a village, using "sticks and batons as well as knives and machetes."

"Men, women and children and babies were found in the field. Some bodies were found naked, some had hands tied up, some had their heads crushed. Several bodies were disembowelled or otherwise mutilated," Fremr said.

In other attacks, women were "killed either while resisting rape or after being raped" while people were forced to dig mass graves that they were later killed and thrown into.

- 'Victory against impunity' -

Ntaganda was found guilty of personally shooting dead a Catholic priest and as an "indirect perpetrator" for the other crimes by orders to target civilians.

These included recruiting child soldiers who "wore uniforms that were often too large for them" but nevertheless were "punished and suffered physical violence" in the same way as adult soldiers," the judgment said.

The rape of both female and male members of his own forces also took place, including three girls under the age of 15, the judges said.

Formerly a Congolese army general, Ntaganda then became a founding member of the M23 rebel group, which was eventually defeated by Congolese government forces in 2013.

Later that year he became the first-ever suspect to voluntarily surrender to the ICC, when he walked into the US embassy in the Rwandan capital Kigali.

Ntaganda's former FPLC commander, Thomas Lubanga, was sentenced to 14 years in jail in 2012. In all, five Congolese warlords have been brought before the ICC.

But the court's prestige has suffered a string of setbacks over recent years with some of its most high-profile suspects walking free, including Ivorian former leader Laurent Gbagbo earlier this year.

The US administration of President Donald Trump has also attacked the court after warning it against prosecuting US service members over alleged war crimes in Afghanistan.

The International Federation of Human Rights (FIDH) however hailed the court, saying the Ntaganda judgment was "an important victory against impunity" and a "day of joy for Ituri victims."

© Agence France-Presse