The Britons were among three people abducted by unidentified armed men while reportedly visiting Virunga National Park, a renowned gorilla sanctuary in the east of the country.
A female ranger travelling with them was killed and their driver was also taken captive, a park spokesman said.
Their vehicle was ambushed north of the city of Goma, the capital of North Kivu province, on Friday.
Major Guillaume Kaiko Ndjike, the Congolese army’s spokesman in North Kivu, said: “With Virunga National Park being within our zone of action, we have joined the park rangers for search operations for the people taken hostages.”
The park has experienced an increase in violence in recent months.
Armed groups have staged a series of raids to steal resources, particularly charcoal.
Last month, five young rangers and a driver in the park were killed in an ambush.
It was the deadliest attack in recent years and took the total number of rangers killed to 175.
Virunga is a UNESCO world heritage site. It covers 3,000 square miles on the country’s border with Uganda and Rwanda.
More from World
Polls open in first Iraq election since victory declared over Islamic State
Former Malaysian PM slapped with travel ban amid corruption scandal
‘Fake news’ row hits Eurovision ahead of final
US to help N Korea’s economy if it abandons nuclear weapons
May challenges Trump on impact of Iran sanctions in phone call
Palestinians shot dead as ‘volatile’ Gaza border protest intensifies
Founded in 1925, it is home to about a quarter of the world’s critically endangered mountain gorillas.
A Foreign Office spokesman said: “We are in close contact with the authorities in Democratic Republic of the Congo following an incident involving two British nationals, and our staff are providing support to their families.”