Nielsen spoke in El Salvador’s capital during a meeting with her Central American counterparts to discuss a regional security plan to fight illegal human trafficking, organized crime and gangs, and expand intelligence sharing and fortify border security.
She said the region is facing a humanitarian and security crisis, adding that "I want to thank my colleagues for their determination to increase cooperation with respect to caravans."
She called on security ministers from El Salvador, Guatemala and Honduras "to show a bold leadership to stop the formation of new caravans."
In recent months thousands of Central Americans have moved in caravans toward the U.S. border. Those in the caravans repeatedly say they are fleeing violence and poverty and hope to apply for refugee status in the United States.
"The caravans are a new phenomenon," Nielsen said. "We are working together to understand their purpose, their plans, and make sure that those who are vulnerable contained within them are protected."
That was an apparent reference to migrants bringing children along on the caravans, in part because U.S. authorities have to quickly release them.
Related: Reuters - U.S., Central America launch plan to crack down on people smugglers