US Attorney General Jeff Sessions on Tuesday announced that the Donald Trump administration will send more immigration judges to detention centers on the border with Mexico and greater efforts will be made to punish undocumented migrants who reenter the country after being deported.

"For those that continue to seek improper and illegal entry into this country, be forewarned: This is a new era. This is the Trump era," Sessions said at a press conference in the border city of Nogales, Arizona.

"The lawlessness, the abdication of the duty to enforce our immigration laws and the catch and release practices of old are over," he added.

The attorney general reiterated that the Trump administration will pursue the fight against undocumented immigration, as well as against criminal organizations, drug cartels and gangs.

"It is ... here, along this border, that transnational gangs like MS-13 and international cartels flood our country with drugs and leave death and violence in their wake. And it is here that criminal aliens and the coyotes and the document-forgers seek to overthrow our system of lawful immigration," he said.

Sessions said he had written on Tuesday to all the country's federal prosecutors to tell them that prosecuting people who violate US immigration laws will be a priority.

Undocumented foreigners who reenter the country after being deported will face felony charges, "if ... certain aggravating circumstances are present," such as being a gangmember or representing a danger to public safety, he said.

Those who smuggle foreigners into the country also will be harshly punished, Sessions said.

He said that 25 new immigration judges had already been dispatched to detention centers along the border to deal with the backlog of some 534,000 immigration cases and that another 50 will be assigned during the remainder of this year, along with 75 more in 2018.

Sessions also emphasized the "success" that the new immigration directives implemented by Trump had had so far, adding that there has been a 40 percent reduction in undocumented migrants crossing the southern border between January and February, when 18,754 arrived, and March, when 12,193 arrived, the lowest figure in 17 years.

"This is no accident. This is what happens when you have a President who understands the threat, who is not afraid to publically identify the threat and stand up to it," Sessions said.