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EXCLUSIVE: Republicans on the House Oversight Committee on Wednesday wrote to DHS Secretary Alejandro Mayorkas objecting to what they described as "silence" from the administration about the crisis at the southern border and the plan to tackle it.

"We still do not know what the administration’s plan is, if any, to reduce illegal border crossings by [unaccompanied children] and family units along the southern border," the letter from 20 Oversight Republicans, led by ranking member James Comer, R-Ky., says.

BIDEN ADMINISTRATION LETS CHILD MIGRANT CENTERS EXPAND TO 100% CAPACITY AMID SURGE IN NUMBERS

The lawmakers write that they do not know what the administration is doing to prevent COVID-19 infections and its spread into border communities as migrants are released into the interior from DHS custody.

"The administration’s silence in this matter can only be understood as confirmation that the administration does not have a coherent plan to respond to this crisis," the letter, obtained by Fox News, says.

The committee Republicans had written to the administration last month requesting documents and information about the crisis -- specifically what is the administration’s plan to reduce illegal border crossings, what it is doing to stop the spread of COVID at ports, how it is seeking to stop the release of migrants into the interior, and the steps it is taking to address the situation with Mexico.

But, they say, they never received anything more than an interim response acknowledging receipt: "Our questions remain unanswered."

The Biden administration has been under heavy criticism from Republicans for its handling of the crisis, particularly its denial that there is a crisis at all. Mayorkas last week denied there was a crisis at the border.

"I think that the -- the answer is no. I think there is a challenge at the border that we are managing, and we have our resources dedicated to managing it," he said.

DHS CHIEF REQUESTS VOLUNTEERS TO HELP AT SOUTHERN BORDER AMID 'OVERWHELMING' MIGRANT SURGE 

Meanwhile, numbers have been surging at the border, and agencies have been struggling to cope.

A senior DHS official confirmed to Fox News that the number of migrant children in custody along the border has tripled in the past two weeks to more than 3,250 and that more than 1,360 have been held for longer than the allowed three days. The numbers were first reported by The New York Times.

Multiple sources told Fox that numbers to be released this week will show about 100,000 apprehensions in February, higher than in February 2019 at the beginning of that year’s border crisis.

The administration is allowing child migrant facilities to expand to 100% capacity as it looks to cope with the numbers, while it is opening other facilities -- including looking at using a Virginia military base.

Meanwhile, Mayorkas this week urged DHS staff to volunteer to help border agencies cope with the "overwhelming" migrant numbers at the border.

Republicans have also criticized the Biden administration rollbacks of a number of Trump-era enforcement and border protections, saying it is incentivizing migration from countries south of the border. GOP House Leader Kevin McCarthy will next week lead a delegation to the border. 

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The Oversight Republicans on Wednesday warned that the growth in unaccompanied alien children (UACs) and family units risks overwhelming border facilities, "presenting a risk to DHS personnel and potentially leading to widespread COVID-19 infection and fatalities in border communities and within the interior of the country."

"The situation is becoming untenable every day that passes without a coherent plan to address this crisis," they added.

Fox News' Griff Jenkins contributed to this report.