Twenty Republican governors are urging the Biden administration to take further action on the crisis at the southern border as the question of where to house the surge in unaccompanied children is impacting states -- with the governors describing the situation as "unacceptable and unsustainable."

"The crisis is too big to ignore and is now spilling over the border states into all of our states," the letter to President Biden, from governors from states including South Dakota, Georgia, Iowa, Nebraska and New Hampshire, says.

Officials at the border encountered more than 172,000 migrants in March, including a record number of unaccompanied children numbering at more than 18,000. Similar numbers are expected for April and could rise in the summer months.

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The Biden administration has touted its ability to empty overwhelmed Border Patrol facilities, where migrants were pictured packed side-by-side in facilities, critics have noted that the migrants have only been shifted in many cases to Health and Human Services (HHS) facilities -- where numbers have skyrocketed from 11,000 in March to 22,000 in May.

The governors say that HHS is calling on states to identify housing locations, while also circumventing them by working with NGOs (non-governmental organizations) and private organizations to house them in their shelters in those states.

"Allowing the federal government to place a potentially unlimited number of unaccompanied migrant children into our states’ facilities for an unspecified length of time with almost zero transparency is unacceptable and unsustainable," they write. "We have neither the resources nor the obligation to solve the federal government’s problem and foot the bill for the consequences of this administration’s misguided actions."

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The administration has emphasized the "root causes" of the crisis, like poverty and violence in Central America, while also blaming the Trump administration for allegedly not being adequately prepared for a migrant surge of the kind seen in the early months of this year.

Critics, however, have pointed to the rollback by Biden of key Trump policies and agreements like the Migrant Protection Protocols (MPP) -- which kept migrants in Mexico to await their hearings -- and asylum cooperative agreements with Northern Triangle countries. They have also blamed increasingly friendly rhetoric from the administration which they say has encouraged migrants to come north.

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"The cause of the border crisis is entirely due to reckless federal policy reversals executed within your first 100 days in office," the letter says. "The rhetoric of the Biden administration and the rollback of critical agreements with our allies have led to the inhumane treatment of tens of thousands of children and undermined a fragile immigration system."

After also citing the increase in drugs seized across the border, the governors urge Biden "to take action to end the humanitarian crisis and secure our southern border immediately."

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Republicans have been ramping up pressure on the administration. While Vice President Kamala Harris is due to visit Guatemala and Mexico next month as part of her efforts to tackle the root causes, Sen. Bill Hagerty, R-Tenn., made a similar trip last week -- and emphasized what he described as the urgency of the current situation.