A pair of US judges who were recently sent to the Mexican border to process asylum requests from migrant women and children have reportedly been withdrawn from their posts because they had to few cases to handle.

Deployed in an effort to quickly hear the claims of migrants seeking asylum so that those deemed ineligible could be deported, the judges left behind scheduled hearings in their home courts.

But after they arrived at detention centers housing women and children in Dilley and Karnes County, Texas, they had so few cases their presence was deemed a waste of resources by the US Department of Justice, a source told the Reuters news agency.

There has been a sharp drop in illegal border crossings by women and children since US President Donald Trump took office in January.

The number of parents and children apprehended at the border dropped to just over 1,000 in March, a 93 percent fall from December, the Department of Homeland Security reported last week.

The number of people apprehended by border patrols while trying to cross into the US, also fell for the fifth consecutive month to their lowest level since 2000.

Eight immigration judges were reassigned from their regular courts to detention centres at the border beginning on March as part of Trump's executive order to curb illegal immigration.

While six have had full dockets, handling dozens of cases per week, the judge in Dilley had no hearings and the judge in Karnes County had four, according to a spokeswoman for the US Department of Justice's Executive Office of Immigration Review.

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The Trump administration was quick to claim the immigration figures showed its tougher stance on immigration had been a success.

Speaking on his first visit to the US-Mexico border, Attorney General Jeff Sessions said the US government would crack down on the "filth" of immigration offences.

The American Civil Liberties Union, which has fought legal challenges on behalf of undocumented migrants, has strongly condemned harsh new controls on immigration.

The ACLU said in a statement: "The Trump administration is intent on inflicting cruelty on millions of immigrant families across the country.