The migrant crisis at the U.S.-Mexico border is one that "is not going to be fixed overnight," according to Vice President Kamala Harris, who last month was appointed by President Biden to manage the U.S. response to the crisis.

Since receiving her border-related assignment more than three weeks ago, Harris has spoken with the leaders of Mexico and Guatemala – and is making plans to visit both countries soon.

But Harris has not yet spoken with the leaders of El Salvador or Honduras, two countries that, along with Guatemala, make up the so-called "Northern Triangle" of Central America, according to the Los Angeles Times.

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The vice president’s lack of communication with the two leaders appeared to undermine White House claims that Harris has been focused on the Northern Triangle nations and the "root causes" of the migrant crisis.

That argument emerged as Harris began to face increasing criticism from Republicans for not making visits to U.S. border regions in California, Arizona, New Mexico and Texas – and for failing to hold a news conference about her border crisis-related duties.

Harris said that working with the Northern Triangle leaders would help address "the most intractable issues" behind the migrant crisis, including poverty, hurricanes, drug cartels and gang violence, the Times reported. Those factors have been considered key reasons why many people from Central America choose to leave their home countries and attempt to enter the U.S.

Perhaps complicating matters for Harris: The brother of Honduran President Juan Orlando Hernandez was sentenced to life in prison last month in New York City after being convicted on U.S. drug charges.

The brother, Juan Antonio "Tony" Hernandez, a former Honduran congressman, was also ordered to hand over $138 million, The Associated Press reported.

A source described as a "senior adviser" to Harris said the Biden administration may attempt to bypass Hernandez and negotiate instead with lower-level officials or nongovernment organizations in Honduras, according to the Times.

Another potential obstacle is Central America’s long history of corruption and violence, the Times noted.

"There are whole big swaths of El Salvador that are essentially being run by violent gangs," U.S. Rep. Zoe Lofgren, D-Calif., said at a congressional hearing Wednesday, according to the newspaper. "It was described to me as like [the Islamic State] without the religion."

Harris told the Times that the Biden administration needs to build "incentives for investment" in the Central American nations so their citizens remain there.

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Earlier this month, Reuters reported the Biden administration was considering a "conditional cash transfer program" to provide economic aid to the Central American nations but Republicans slammed the proposal as "madness."

"President Biden wants to try and buy our way out of this border crisis with taxpayer money," House GOP Leader Kevin McCarthy, R-Calif., wrote on Twitter. "The Administration is already spending $60 million a week, and now wants to launch a cash transfer program in Central America. This insults millions of Americans who are out of work in our country."

At a Northern Triangle security roundtable session Wednesday, Harris said she had plans to travel to Mexico and Guatemala "as soon as possible" in hopes of advancing her diplomatic efforts.

The Associated Press contributed to this story.