By Mark Hosenball and Jonathan Allen

WASHINGTON/NEW YORK (Reuters) - The Obama administration on Thursday came under fire from Democratic presidential candidates and human rights groups for plans to intensify deportations of Central American migrants by rounding up undocumented families.

Amid a surge in the arrival of unaccompanied children and families from El Salvador, Guatemala and Honduras, U.S. government sources confirmed preparations to detain and deport, starting next month, families that already have been ordered to leave.

The Washington Post first reported on Wednesday that the Department of Homeland Security was considering launching the crackdown in January.

Government sources, who asked not to be identified, said on Thursday the campaign by DHS's Immigration and Customs Enforcement (ICE) marks an expansion from mostly targeting individuals to pursuing families with undocumented members.

A spokesman for Hillary Clinton, who is leading among Democratic candidates seeking the presidency in next November's elections, said she "has real concerns about these reports," adding it is "critical that everyone has a full and fair hearing and that our country provides refuge to those that need it."

An ICE spokesperson, asked why the agency was targeting families, said it focuses on people who pose a threat to national security, public safety and border security, "whether alone or with family members."

Senator Bernie Sanders, chief rival to Hillary Clinton for the Democratic presidential nomination, said in a statement, "Our nation has always been a beacon of hope, a refuge for the oppressed ... we need to take steps to protect children and families seeking refuge here, not cast them out."

Another of Clinton's Democratic opponents, ex-Maryland Governor Martin O'Malley, tweeted: "Holiday plans for raids to round up/deport Central American refugees fleeing death are wrong. We are a better nation than this."

Many seek asylum in the United States by claiming their governments are unable to protect them from drug-related or domestic violence.

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Support for the deportations came from Republican Senator Ron Johnson, who chairs the Homeland Security and Government Affairs Committee. "The primary solution to reducing this ongoing crisis is to eliminate the incentive that results from allowing 95.6 percent of these illegal immigrants to stay - by humanely and expeditiously returning them to their home countries."

DHS has put a priority on targeting for deportation previously convicted criminals who are undocumented. But it also wants to remove recently arrived Central Americans.

Hispanic-Americans are a growing force in U.S. politics and Democrats hope a strong turnout in November could help them, especially with Republicans campaigning on a vow to seal U.S. borders from illegal immigrants.

Donald Trump, who leads the Republican presidential field, caused an uproar last June, accusing Mexico of sending rapists and other criminals into the United States and saying undocumented immigrants carried "tremendous infectious disease."

Frank Sharry, executive director of the pro-immigration group America's Voice, said the upcoming raids would "be a political nightmare for the Democrats."

Michelle Brané, director of the Women's Refugee Commission's migrant rights program, said she had not received briefings from the administration. But she worried that some families facing deportation may not have had proper legal representation, especially if their cases sped through the courts.

"It's a pretty traumatic way to break in the new year," she said.

(Additional reporting by Julia Edwards in Honolulu; Writing by Richard Cowan; Editing by Richard Chang and Steve Orlofsky)