Militants killed 35 civilians in an attack on an artisanal gold mining site in the Democratic Republic of Congo's eastern province of Ituri, the presidency said on Monday.

Rebel groups in the central African country's mineral-rich east have been wrangling over land and resources for the past decade, a conflict that has claimed thousands of lives and displacing millions in the process.

On Sunday, fighters linked to the CODECO militia raided a mining encampment near the town of Mongwalu, where they have been competing with another rebel group for access to the site, army spokesperson Jules Ngongo Tsikudi said.

"People were caught in the cross-fire... many lost their lives and their homes [were] burnt," he said in a statement.

Congo's presidency said on Twitter that CODECO had "assassinated" 35 civilians during the attack.

Civil society leaders estimated that more than 50 people were killed. One said a local hospital had been overwhelmed by the number of victims.

Reuters was not able to reach CODECO for comment.

The group has built a reputation for targeting civilians, killing 18 people at a church last month and another 60 at a displaced persons camp in February.

Violence is a daily occurrence in Congo's vast eastern provinces, where groups like CODECO skirmish with other militias including a local Islamic State affiliate.

Congo's government declared martial law in Ituri and neighbouring North Kivu province a year ago in an effort to quell the violence. But deadly raids have only surged since then, according to the Kivu Security Tracker, which monitors conflict in the region.