Also, five American troops died Saturday in bombings in the south where international forces are stepping up the fight against the insurgents.
A NATO statement Saturday said the two service members left their compound the previous day in Kabul but did not return.
The statement did not identify the pair by nationality but U.S. officials said they were American.
The officials spoke on condition of anonymity because of the sensitivity of the information.
The military has dispatched vehicles and rotary-winged aircraft to search for them and their vehicle.
Local official: 'They came down the wrong road'
Samer Gul, district chief of Charkh district in Logar province, said Saturday that a four-wheel drive armored vehicle was seen Friday night by a guard working for the district chief's office. The guard tried to flag down the vehicle, carrying a driver and a passenger, but it kept going, Gul said.