Islamic State-backed rebels killed 38 people on Sunday in an attack on a church in eastern Congo, city officials said.

The assault in Komanda, a city in the northeastern Democratic Republic of Congo, is believed to have been carried out by ADF rebels, wielding guns and machetes, officials told Reuters.

Jean Kato, an official in the city administration, said worshippers were taking part in a night mass when the rebels stormed the church in the early hours of Sunday morning.

Thirty-eight people were dead, 15 injured and several others were still missing, officials said.

Christophe Munyanderu, a human rights activist present at the scene in Komanda, said shots were heard overnight but people at first thought it was thieves.

"The rebels mainly attacked Christians who were spending the night in the Catholic church," said Munyanderu.

"Unfortunately, these people were killed with machetes or bullets."

The United Nations Organisation Stabilisation Mission in the DR Congo has condemned a recent resurgence in violence in the province where this attack happened.