Joel Wengamulay, spokesman for the Congolese Institute for The Conservation of Nature, advised the United Nations -backed Radio Okapi that the kidnapping occurred Friday within the Nyiragongo territory simply north of Goma.
Local media reports say the ranger shot dead was female and acting as a guard, while the United Kingdom citizens were taken along with their Congolese driver.
Congolese soldiers and park rangers are searching for two British tourists kidnapped in the country's volatile eastern borderlands, an army spokesman said.
More than 175 rangers have died protecting Virunga National Park, which is located in the rugged mountains and volcanic plains adjacent to neighouring Rwanda and Uganda.
De Merode, a Belgian national, was himself wounded in a road ambush between the park and Goma, the capital of North Kivu, in May 2014.
The Foreign Office said it was in close contact with the authorities in the DRC and was supporting the families.
The Press Association reported that the two tourists were British citizens.
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Virunga is home to about a quarter of the world's population of critically-endangered mountain gorillas, as well as to eastern lowland gorillas, chimpanzees, okapis, lions, elephants and hippos.
There have been a number of killings and kidnappings in recent years.
Last month, five young rangers and a driver were killed in a militia ambush, the park said.
Last year, a fifth of the park's southern sector was deforested owing to illegal charcoal production, the park said.
The Foreign Office has issued warnings not to travel to large parts of the DRC.
It added that British government staff are not always in the area and the British embassy's ability to offer consular assistance may be "severely limited".