WASHINGTON — President Barack Obama has authorized the Defense Department to call up a small number of National Guard or reserve troops that possess special skills needed to aid efforts in stopping the spread of the deadly Ebola virus in West Africa.

Obama issued an executive order last week authorizing the secretaries of Defense and Homeland Security to order the Selected Reserve and certain members of the Individual Ready Reserve to deploy to West Africa, where as many as 4,000 U.S. troops are headed, most of them to Liberia, to support U.S. and international efforts to stop the rapidly spreading virus that has killed nearly 4,500 people.

U.S. officials say Defense Secretary Chuck Hagel requested the order after determining that specialists with skill sets needed by Operation United Assistance, including engineers, comptrollers and religious specialists, were in short supply, or to replace active-duty personnel.

Already, elements of the Kentucky Air National Guard are in Dakar, Senegal, to establish a staging base for the Liberia-based mission, having volunteered and deployed before the presidential order was issued.