A senior military source said on Friday that an attack was conducted overnight near the town of Eringeti in North Kivu province, which borders Uganda. Fighting continued until Friday morning, the source added.
The attack also left 20 Congolese soldiers injured, said the source, who spoke on condition of anonymity as he was not authorized to speak to the media.
The armies of Congo and Uganda launched a military operation late last year against the Allied Democratic Forces (ADF), accused of attacking a UN base on December 8 and killing 15 peacekeepers.
"The combat continues and our goal is to recapture all the ADF positions in all strategic locations to keep them from organizing," said the senior officer.
The attack was confirmed by a top Congolese security official and a foreign diplomat, who put the death toll at 22 Congolese soldiers.
"We are at war. And when you are at war, ambushes are possible. That does not surprise me," Congolese Defense Minister Crispin Atama Tabe told journalists in the capital Kinshasa when asked about the assault.
Dozens of armed groups have been active in the eastern DR Congo, long after the official end of a 1998-2003 war during which millions of people died, mostly of hunger and disease.
The United Nations has warned that the Democratic Republic of the Congo was at a "breaking point," saying this year it will seek more than $1.5 billion to respond to the worsening humanitarian crisis in the country.
The escalating violence in Congo’s center and east this year comes amid a political crisis in the country due to President Joseph Kabila's refusal to leave office after his term expired in December 2016.