One ranger was killed during the kidnapping of two British tourists in the Democratic Republic of Congo (DRC).
A female park ranger was killed during the incident.
He mentioned that "my ideas are actually with the household of Virunga Park ranger Rachel Makissa Baraka who was killed in the course of the kidnapping, and with the injured driver".
A spokesperson for the Congolese Institute for Nature Conservation (ICCN) said: "For the moment the [ICCN] can not communicate much about the incident because the hostages are still in captivity".
Mr Johnson said his thoughts were with Ms Baraka's family, the injured driver, "and the released British nationals as they recover from this traumatic incident".
Bethan Davies and Robert Jesty were among three people abducted by unidentified armed men while visiting Virunga National Park, a renowned gorilla sanctuary in the east of the country.
The tourists' vehicle was ambushed while transporting them from the park about 10 kilometres (6 miles) north of the city of Goma, a park guard told The Associated Press.
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"We want to convey our deepest condolences to her family and our honest gratitude for her courage in the service of Congo".
The park's director, Emmanuel de Merode, said: "Ranger Baraka's life was tragically cut short in service to Virunga National Park".
Although a number of Congolese nationals have been abducted in the park in recent years, it is the first time in almost 20 years that an attempt has been made to seize Western tourists, reports quoted local DRC officials as saying. Virunga Park is home to about one-quarter of the world's remaining mountain gorillas, and the work of protecting them has proven risky.
But it is located in DR Congo's North Kivu province, where armed groups are fighting for control of territorial and natural resources, and where poaching is a major threat.
"This sad total once again illustrates the risks that park wardens take every day to protect wildlife and the waterside communities".
Five Virunga rangers and a staff driver were killed just last month during an ambush in the central sector of the park.