WASHINGTON — The White House on Tuesday announced a substantial expansion of a program to admit Central American refugees to the United States, conceding that its efforts to protect migrants fleeing dangerous conditions has been inadequate and left too many vulnerable people with no recourse.

Currently, the program allows unaccompanied Central American children to enter the United States as refugees. It will be expanded to include their families, permitting siblings over the age of 21, parents, and other relatives who acted as “caregivers’’ to qualify.

Officials could not say how many refugees might be eligible under the expansions, but the change is a potentially significant one, essentially opening a new channel for Central American families to gain legal entrance to the United States.

“Our current efforts to date have been insufficient to address the number of people who may have legitimate refugee claims,’’ said Amy Pope, a deputy homeland security adviser.

The White House also said it had reached an agreement with Costa Rica to serve as a temporary host site for the most vulnerable migrants from El Salvador, Guatemala, and Honduras while they wait to be processed as refugees, once they have undergone security screening in their home countries. The UN high commissioner for refugees has agreed to set up an unusual process for reviewing requests for people in their home countries to qualify as refugees and send them to Costa Rica if they are facing immediate danger.

“It shows the administration now recognizes this is primarily a refugee flow, not an economic one,’’ said Kevin Appleby, senior director for international migration policy at the Center for Migration Studies in New York.

New York Times