US President Barack Obama Sunday mobilized military reserves to help quake-hit Haiti, particularly medical staff to work on hospital ships and Coast Guard staff to secure the ports.

In a presidential order, Obama wrote it was "necessary to augment the active Armed Forces of the United States for the effective conduct of operational missions, including those involving humanitarian assistance, related to relief efforts in Haiti."

He said he was authorizing Defense Secretary Robert Gates and Homeland Security Secretary Janet Napolitano to order to active duty any units of the reserve as needed.

The order would help both departments to support the massive US aid operation in Haiti, devastated by a 7.0-magnitude quake on Tuesday.

"It will be used on a limited basis, principally to support targeted functions associated with this humanitarian mission," the White House stressed in a statement.

"For instance, it will allow for the call-up of reserve medical personnel to replace those deployed on the hospital ship Comfort and it will allow for the deployment of a Coast Guard unit that will help provide port security."

Obama also sent a letter to House Speaker Nancy Pelosi informing Congress of the move.