GOMA, Democratic Republic of Congo (Reuters) - At least nine people died in clashes in eastern Democratic Republic of Congo that erupted when government soldiers prevented former fighters from leaving a camp for demobilized militia groups, officials said on Thursday.

The camp houses ex-fighters from the Rwandan-backed M23, which participated in rebellions against the government of President Joseph Kabila in 2012 and 2013, and the Mai Mai militia, whose allegiance fluctuated during the conflicts, a U.N. official said.

Emmanuel Cole, president of the Bill Clinton Foundation for Peace, a human rights NGO in Kinshasa, said six ex-militia fighters and three soldiers died in Wednesday's clashes.

Congo's east has been plagued by instability and conflict for two decades. The region suffered the worst of the fighting during a war between 1996 and 2003 that sucked in half a dozen African countries and left millions of people dead.

World powers fear a repeat of that violence as Congo approaches a contentious election period with Kabila, in power since 2001, accused of seeking to delay the poll or change the constitution to extend his mandate for another term. Kabila has not commented.

One of the former militiamen, who spoke on condition of anonymity, said ten of them were killed and that a demonstration they held to demand to be allowed to go home was peaceful.

Government spokesman Lambert Mende acknowledged that the two groups had clashed but denied that soldiers had killed anyone. He said protesters killed one camp officer with a machete.

The incident also underscored existing tensions in a number of camps for former militia fighters across Congo, which rights groups criticize for inhumane conditions.

New York-based Human Rights Watch said in 2014 that more than 100 people died from starvation and disease at one camp in northwest Congo owing to inadequate food and medicine.

(Story corrects paragraph 5 to show Kabila came to power in 2001, not 2002.)

(Reporting by Aaron Ross; Editing by Tim Cocks and John Stonestreet)