(Adds date of first court appearance; previous KINSHASA)

THE HAGUE, Feb 7 (Reuters) - A former Congo warlord was arrested and taken to the International Criminal Court in The Hague on Thursday to face war crimes charges including murder, sexual slavery and using child soldiers, the court said.

Mathieu Ngudjolo was the head of the Front of Nationalists and Integrationists (FPI) militia during conflict in northeast Ituri Province that grew out Congo's 1998-2003 war.

"With this arrest again, we are showing that there can be no impunity for massive crimes," Deputy Prosecutor Fatou Bensouda said at a hearing in The Hague after Ngudjolo was taken into custody there.

He was arrested by Congolese authorities on Wednesday and handed over to ICC custody.

The third Ituri warlord to be transferred to the ICC, Ngudjolo is charged on three counts of crimes against humanity and six counts of war crimes. His first court appearance is scheduled for Monday, Feb. 11, the court said in a statement.

"(Ngudjolo) attacked primarily the Hema ethnic group with the participation of children under the age of 15," Paul Madidi, a spokesman for the court in Kinshasa said. "He is responsible for the murders of 200 civilians, looting, and reducing women and girls to sexual slavery."

Another Congolese militia chief, Thomas Lubanga, was taken into custody by the court in 2006 and his trial is due to start on March 31. He is accused of recruiting children under the age of 15 to kill members of another ethnic group.

The ICC is also in the process of prosecuting Germain Katanga, another Ituri ex-militia leader who is accused of murder, sexual slavery and using child soldiers.

Ngudjolo's arrest comes as the government of President Joseph Kabila is trying to end a decade of violence in Congo that experts estimate has killed 5.4 million people, mainly through hunger and disease.

Last month, the government signed peace deals with 25 armed groups, including renegade General Laurent Nkunda's Tutsi insurgency, in a bid to finally stabilise the east, where fighting has continued despite the end to the broader war.

CHILD SOLDIERS

Ngudjolo was one of three Ituri warlords to sign a similar peace agreement with the government in 2006, promising to disarm in return for officers' commissions in the Congolese army.

He was officially made a colonel as part of the deal and flew to Kinshasa late last year to begin officer training. Unlike Lubanga and Katanga, he was not being held in detention by Congolese authorities when he was handed over to the ICC.

"Ngudjolo's arrest shows that justice will reach those who seem untouchable because of their official position," Param-Preet Singh, a lawyer with New York based Human Rights Watch (HRW), said in a statement on Thursday.

The Ituri conflict, which raged well after a wider peace accord officially ended the 1998-2003 war, pitted foreign-backed militias from the Hema and Lendu ethnic groups against one another. The fighting in Ituri killed over 70,000 people.

While welcoming the move to send Ngudjolo to The Hague, HRW's Singh said the court must extend its investigation into crimes into Ituri to include those responsible for backing the warlords both militarily and financially.

"The three rebel leaders held by the ICC did not act alone in terrorising civilians in Ituri," she said.

"The ICC prosecutor should investigate their links to officials in Congo, Rwanda, and Uganda, who might also be responsible for atrocities." (Reporting by Joe Bavier in Kinshasa; additional reporting by Reed Stevenson in Amsterdam) (Editing by Pascal Fletcher and Charles Dick)

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