WASHINGTON — President Joseph R. Biden, Jr., on Wednesday named Vice-President Kamala D. Harris to lead US efforts with Mexico and Central America’s Northern Triangle countries to try to stem the flow of migration to the United States.

Mr. Biden’s decision gives a high-profile assignment to his vice-president, a daughter of immigrants who has forged a reputation as an ally of immigration advocates. As California attorney general, Ms. Harris had to deal with a major influx of unaccompanied minors at the state’s border with Mexico in 2014.

It is a task that carries political risks for Ms. Harris, a potential future presidential candidate. Border woes have been an intractable problem for multiple presidents.

Mr. Biden served in a similar role for then-President Barack Obama when he was vice-president. By assigning her to handling diplomatic efforts with Central America, Mr. Biden is elevating the migration issue as a top priority.

Just two months into office, Mr. Biden is struggling to get a handle on a burgeoning migration challenge along the US border with Mexico—a problem the Democrat blamed on “the somewhat draconian” policies of his Republican predecessor, Donald J. Trump, who left office with his border wall incomplete.

Mr. Biden said the United States was going to need help from Mexico and the Northern Triangle countries of Honduras, Guatemala, and El Salvador and that Ms. Harris “agreed to lead our diplomatic efforts and work with those countries.”

“The best way to keep people from coming is to keep them from wanting to leave,” Mr. Biden said, listing gang violence, drug-trafficking cartels, hurricanes, floods, and earthquakes as factors spurring migration.

Ms. Harris said the job “will not be easy, but it is important work, it is work that we demand as a people of our country.”

US officials are battling to house and process an increasing number of unaccompanied children, many of whom have been stuck in jail-like border stations for days while they await placement in overwhelmed government-run shelters.

Senior administration officials said Ms. Harris’s focus would be on regional solutions and working with leaders in the region to make it safer for people to remain in their home countries and make asylum requests there.

“We’re going to look at the possibility for people in their home countries in the Northern Triangle to have a predictable, regular process of seeking asylum there so they don’t have to take this phenomenally dangerous journey or, worse yet, send their children unaccompanied,” said one senior official who briefed reporters on the Harris move.

Ms. Harris is expected to travel to the region at some point, but no trips are planned yet, another official said. Phone calls with leaders in the region are expected as well.

El Salvador President Nayib Bukele criticized the US focus on Northern Triangle countries in a tweet on Wednesday, apparently seeking to decouple his country from Honduras and Guatemala in US policy.

Mr. Bukele retweeted a chart with data indicating a significant decrease in the amount of unaccompanied children from El Salvador taken into custody by US Customs and Border Protection since 2020, compared with previous years.

“If the United States wants to seriously address immigration at the southern border, they should really drop the ‘Northern Triangle’ concept,” Mr. Bukele tweeted. “A recycled plan that did not work in 2014 will not work now.”

Since taking office in January, Ms. Harris has been carving out a role for herself as a promoter of Biden’s US coronavirus relief bill, the first major legislation the president signed into law. She has also taken an active role in encouraging Americans to get vaccinated.

While she has not had a specific policy portfolio until now, she has had calls with foreign leaders, including those of allies such as France and Israel. Sergio Gonzales, executive director of the Immigration Hub, a migration advocacy group, welcomed Ms. Harris’s appointment, saying it “underscores the seriousness of the Biden-Harris White House to tackle every aspect of our broken immigration system.” — Andrea Shalal, Steve Holland, and Nandita Bose/Reuters