Three Democratic Republic of Congo soldiers died on Monday while repelling an attack in the eastern Beni region by ADF Ugandan Islamist rebels, who are suspected of murdering 14 UN peacekeepers last month.
The army had on Saturday announced an offensive against the Allied Democratic Forces, one of a number of armed groups acting in North Kivu and South Kivu — the two provinces which border Uganda, Rwanda, Burundi and Tanzania.
"The ADF attacked our position in Muzambay at 4:00 am (0200 GMT)," said army spokesman Captain Mak Hazukay.
"Our forces pushed them back after heavy fighting," he said, but denied comment on casualties.
But a witness told AFP he saw the corpses of three soldiers in the morgue of Beni's general hospital. He saw four other troops being treated for wounds at the same facility.
Another source reported seeing an army ambulance transporting two bodies and five wounded soldiers to hospital.
"The firing started very early while we were still sleeping but stopped around 6:30 am," said Aimee Makinda, the wife of a soldier whose house is located near the site of the clashes.
An AFP correspondent in Beni reported hearing heavy and light weapons fire for two and a half hours from 4 am.
Present in DR Congo since 1995, the ADF was created by Muslim radicals to oppose Ugandan President Yoweri Museveni's rule.
Congolese authorities and the UN mission in DR Congo, MONUSCO, accuse the group of killing more than 700 civilians as well as combattants in the Beni region since 2014.
The group also stands accused of killing 14 UN peacekeepers in eastern DR Congo last month, the biggest single loss of peacekeepers in nearly a quarter of a century.