Central American immigrants await transportation to a U.S. Border Patrol processing center after crossing the Rio Grande from Mexico into the U.S. last year. (John Moore/Getty Images)

Homeland Security Secretary Jeh Johnson announced Monday that federal immigration authorities apprehended 121 adults and children in raids over the New Year’s weekend as part of a nationwide operation to deport a new wave of illegal immigrants.

The families taken into custody by Immigration and Customs Enforcement agents were living in Georgia, Texas and North Carolina, Johnson said in a statement. They are being held temporarily in federal detention centers before being deported to Central America.

“As I have said repeatedly, our borders are not open to illegal migration,” Johnson said. “If you come here illegally, we will send you back consistent with our laws and values.”

[U.S. plans raids to deport families who have surged across the border]

The raids were the first in a broad operation by the Obama administration that is targeting hundreds of families for deportation who have crossed the southern U.S. border illegally since the start of last year. The operation, first reported by The Washington Post, is the first large-scale effort to deport families fleeing violence in Central America, authorities said.

Homeland Security officials declined Monday to say how many children were among those apprehended, where they were found and what countries they were fleeing. The operation is targeting only adults and children who have already been ordered removed from the U.S. by an immigration judge, officials familiar with the raids have said.

Asked about the removal operations, White House press secretary Josh Earnest said that the administration has “prioritized the cases of recent arrivals, individuals who recently crossed the border without proper documentation. That is the nature of the operations carried out in the last several days.”

Earnest said the efforts are “consistent with the priorities the president himself talked about, deporting felons not families, with a focus on individuals who only recently crossed the border.”

He added that those targeted for removal have gone through due process legal proceedings and said the effort will help discourage Central American families that sending their children on a dangerous journey to try to enter the United States illegally.

“People should take from this the understanding that the administration is quite serious when it comes to enforcing immigration laws.”

Because of the sensitivity of taking children into custody and deporting them and their families, authorities took precautions during the raids, Johnson’s statement said, using female agents when possible to apprehend families and having medical personnel on hand.

ICE also was lenient in a number of cases “for health or other personal reasons,” Johnson said. The agency usually exercises discretion on a case-by-case basis, using a family’s individual circumstance, length of time in this country and ties to the community, Homeland Security officials said.

More than 100,000 families with  both adults and children have made the journey across the southwest border from Mexico, Guatemala, Honduras and El Salvador since last year, in addition to a related surge in unaccompanied children. People familiar with the raids said before Christmas that authorities expected to apprehend a fraction of these migrants in the raids, which are scheduled to continue in coming weeks.

Johnson said the raids follow a crackdown on illegal immigration that started in summer 2014 and has resulted in about 14 flights a week carrying those who have been apprehended back to Central America. But most of those deported were single adults, not children who started flocking across the southern border by themselves in 2014.

Most public attention has focused on these unaccompanied children, whose numbers have spiked in recent months. But the number of  children arriving with at least one family member — known as “family units’’ in DHS parlance — also spiked dramatically in the last year. Illegal immigration has been a major issue in the 2016 presidential campaign, with GOP contenders calling for stricter border patrols.

Advocates expressed concern before Christmas about the raids, citing what they called flaws and abuses in the government’s treatment and legal processing of the families, many of whom are fleeing danger or persecution in El Salvador, Guatemala and Honduras.

Groups that have called for stricter immigration limits have said the raids are long overdue, although they remain skeptical about whether the scale would be large enough to deter future illegal immigration from Central America.

David Nakamura contributed to this report.