KINSHASA (Reuters) - About 80,000 people have fled fighting between the Democratic Republic of Congo army and a new rebel coalition, the United Nations said on Tuesday, joining the millions already uprooted in Africa’s worst displacement crisis.

Militia violence has intensified across Congo since President Joseph Kabila refused to step down at the end of his constitutional mandate in December, raising fears the country will slide back into the wars at the turn of the century that killed millions.

The latest fighting broke out in South Kivu province’s Fizi territory, in the eastern part of the country. Government troops clashed with the National Coalition of the People for the Sovereignty of Congo (CNPSC), a new alliance of local self-defense militias, the U.N. Organization for the Coordination of Humanitarian Affairs (OCHA) said in a new report.

The rebels seized several towns last month before being beaten back by government troops, according to the army, which said at least a dozen people were killed in the clashes.

Conflict has forced more than 1.5 million Congolese to flee their homes this year, far more than in Iraq or Syria. More than 3,000 have died since last October in an insurrection against the government in central Congo’s Kasai region

Altogether, 3.8 million Congolese are internally displaced, more than in any other African country, according to OCHA. Some 7.3 million need humanitarian assistance.

OCHA also warned that tit-for-tat attacks between competing communities in South Kivu risked reviving inter-ethnic tensions that have fueled repeated conflicts.

The conflict in Congo has been aggravated by Kabila’s refusal to leave office. In power since 2001, he was required by the constitution to step down in December, but an election to replace him was postponed after delays registering millions of voters.

Congo’s election commission president said on Sunday that the vote was unlikely to take place in 2017, either, largely due to the violence in Kasai.