Washington (AFP) - President Barack Obama's administration is planning a vast operation to round up and expel from the United States migrant families who fled drought and violence in Central America, reports said on Thursday.

Any such crackdown would be hugely controversial, with immigration one of the hottest topics in the 2016 presidential campaign, and rights groups expressed grave concern at the deportations.

The flow of families and unaccompanied children crossing into the United States from Mexico slowed this year, but the numbers surged upwards again in October and November.

Several Latin American countries are in the grip of violent lawlessness and the El Nino weather pattern has plunged a number of countries in the region into drought.

The Department of Homeland Security did not dispute anonymously sourced reports in The Washington Post and Wall Street Journal that a crackdown is imminent.

According to the reports, hundreds of families living in the United States whose asylum requests have been denied will be rounded up and sent home.

DHS Immigration and Customs Enforcement spokeswoman Gillian Christensen told AFP that Homeland Security Secretary Jeh Johnson "has consistently said our border is not open to illegal immigration.

"If individuals come here illegally, do not qualify for asylum or other relief, and have final orders of removal, they will be sent back consistent with our laws and our values."

Refugee rights activists argue the families are fleeing corruption, gang violence and drought in their homelands and should be treated as refugees.

Republican presidential hopefuls including frontrunner Donald Trump -- who took to Twitter to claim credit for the deportations -- are vying for who can promise the toughest stance on immigration.

Trump tweeted: "Wow, because of the pressure put on by me, ICE TO LAUNCH LARGE SCALE DEPORTATION RAIDS. It's about time!"

In contrast, Hillary Clinton's campaign said the Democratic frontrunner has "real concerns."

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Clinton "believes it is critical that everyone has a full and fair hearing, and that our country provides refuge to those that need it," read a statement reported in US media.

"We should be guided by a spirit of humanity and generosity as we approach these issues."

- 'Regrettable decision' -

In the 2015 fiscal year arrests of non-documented migrants crossing the US southern border dropped by a third to the second lowest level since 1972.

The number of children crossing without their relatives -- which surged in 2014 -- was also well down over the period.

But the number of minors and families crossing began to increase again sharply toward the end of 2015, alarming the Department of Homeland Security.

In El Salvador there was a sobering wave of violence this year. Between January and November there were 6,065 homicides, according to the Forensic Medical Institute.

One of El Salvador's top officials for migration issues called the US decision "regrettable."

"We have been informed about the decision of senior US officials to begin deporting family units and non-accompanied minors," said Liduvina Magarin, deputy minister for Salvadorans abroad.

"It is a regrettable decision by the US government (that affects) our Salvadoran families."

In Honduras, there has been a police crackdown but the homicide rate is still expected to be one of the highest in the world this year at 62 per 100,000 inhabitants.

There was a similar spike in killings in Guatemala. In 70 percent of the cases there, firearms were the cause of death.

Compounding matters, many countries around the Pacific rim are facing extreme weather, including drought. Guatemala, Haiti and Honduras are among the worst victims.