Apprehensions of undocumented migrants on the US border with Mexico soared 70 percent in March to 172,331, hitting the highest level in 15 years, data showed Thursday, in a mounting challenge for the administration of President Joe Biden.

US Customs and Border Protection (CBP) reported that the number of unaccompanied children they detained after crossing the border doubled in March from February to 18,663.

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Another 227 were intercepted by immigration officials at official border crossings, taking to 18,890 the number of migrant minors that the Biden administration has had to accept for resettlement, overwhelming its shelters and processing facilities.

Most of the migrants were from Mexico and the Central American countries of Honduras, El Salvador and Guatemala. CBP said many increasingly arrive in large groups.

CBP said the number of people caught illegally crossing the US border with Mexico in the six months since October was nearly 570,000, up 24 percent from the same period a year ago.

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The agency blamed the surge on “violence, natural disasters, food insecurity, and poverty in Mexico and the Northern Triangle countries of Central America.”

“This is not new,” said Troy Miller, currently acting as CBP commissioner.

“Encounters have continued to increase since April 2020,” he said in a statement.

CBP said nearly 104,000 of the border crossers were expelled back into Mexico, most of them single adults, under rules based on Covid-19 pandemic protections.

But the unaccompanied children and tens of thousands of migrants who arrived in family units with small children have been allowed to stay in the country, creating a large political and social headache for the Biden administration.

Videos show the children jammed into tents and halls built for only a fraction of their numbers.

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They have swamped the ability of the CBP and the Department of Health and Human Services to house them and help them connect with relatives in the United States.

As of Wednesday there were more than 20,000 unaccompanied migrant children in government custody — 4,228 in CBP hands, and 16,045 with HHS, which receives them from the Border Patrol.

The government has set up holding facilities for the children on military bases and is searching for more capacity.

Biden administration officials, in a briefing for reporters, called the surge temporary and blamed the previous government of president Donald Trump for breaking down the government infrastructure for handling the large number of immigrants without papers.

“Nobody should have the expectation that this is going to be solved overnight,” said a senior administration official who would not be identified.

“The president has a plan,” the official said. “We have to build back the system.”

Biden has said that the surge is seasonal and predicted a downturn, but previous years show a pattern of a migrant surge in the early months of the year that doesn’t die off until the border weather gets very hot in May or June.