More than 30,000 people are expected to be housed in shelters as monster storm Harvey continues to devastate the Texas city of Houston and surrounding areas, a top federal emergency official said Monday.

"This shelter mission is going to be a very heavy lift. We're anticipating over 30,000 people being placed in shelters temporarily to basically stabilize the situation and provide for their care," Federal Emergency Management Agency administrator Brock Long told a news conference in Washington.

"Right now this is still an ongoing situation. We're not at recovery yet," he said, adding that authorities were conducting swift water rescues and searching for victims across as many as 50 counties in Texas.