KINSHASA, Democratic Republic of Congo — An American journalist and three Congolese security guards have disappeared in a wildlife reserve in an eastern region of the Democratic Republic of Congo, a day after a local militia attacked a security post there, an official said Saturday.

The assault took place in the Okapi Wildlife Reserve in the eastern Ituri region on Friday afternoon, said the official, Alfred Bongwalanga, the administrator of the Mambasa territory.

The American journalist, a woman who was not identified, had been traveling with two European journalists and 14 security guards, Mr. Bongwalanga said. The group had been trekking in the forest for a few hours, about 25 miles from the nearest city, Mambasa, he added.

Among the 17 members of the group, 13, including the two European journalists, have been found since Friday, Mr. Bongwalanga said. The American journalist and three Congolese guards remain missing.