WASHINGTON, Oct. 16  -- President Barack Obama on Thursday authorized the United States Department of Defense to call up armed forces to active duty to help combat the spread of Ebola in West Africa.

Obama notified Congress of his executive order, saying that "I hereby determine that it is necessary to augment the active Armed Forces of the United States for the effective conduct of Operation United Assistance."

This is providing support to civilian-led humanitarian assistance and consequence management support related to the Ebola virus disease outbreak in West Africa, he said.

According to Department of Defense officials, up to 4,000 military personnel could be dispatched to the region. But the White House said that it didn't know exactly how many reserve troops would be required.

"The Department of Defense has told the president that it will require about 4,000 Department of Defense personnel to execute the mission the president has directed them to execute," White House Press Secretary Josh Earnest said in the afternoon.

Pressure on the White House to combat Ebola has intensified in recent days after a second nurse was diagnosed with the deadly virus.