WASHINGTON - The US military effort to help prevent the spread of Ebola in West Africa could be extended beyond its six-month mission if there is a surge of new cases of the deadly disease, a Pentagon spokesman said Thursday.

Pentagon Press Secretary John Kirby said during a press conference that despite a reported drop in new cases in hardest hit Liberia, it's too soon to determine whether this trend will hold.

"We have to be prepared for this to go longer than six months," he said, which helps to shed light to the decision taken by Defense Secretary Chuck Hagel to authorize the call-up of more than 2,000 reserves and guard.

More than 2,500 US military personnel are in West Africa as part of Operation United Assistance, helping battle Ebola outbreak by providing logistical support and training for health care workers, as well as testing fluid samples and building emergency treatment units.

The World Health Organization says Ebola has killed more than 5, 400 people in Liberia, Sierra Leone and Guinea since March.