Just under 1,000 children -- separated from their parents while attempting to enter the United States during Donald Trump's presidency -- have yet to be reunited, the current US government said Thursday.

When President Joe Biden took over from Trump in January 2021, he vowed quickly to undo his predecessor's so-called "zero-tolerance" approach to illegal immigration, which had resulted in thousands of children being separated from their parents after crossing the US southern border.

A task force set up to coordinate reunification efforts "has to date identified 3,924 children" separated during Trump's four years in office, of which 2,926 have been reunited, the Department of Homeland Security said Thursday.

"Of the 998 children who remain to be reunited... 148 children are in the process of reunification and 183 families have been informed of the opportunity to reunify by a contracted NGO," the press release added.

US Secretary of Homeland Security Alejandro Mayorkas, the leader of the task force, said Thursday that "our critical work is not finished."

"We reaffirm our commitment to work relentlessly to reunite the other families who suffered because of the prior cruel and inhumane policy," Mayorkas said on the second anniversary of the Family Reunification Task Force's establishment.

In 2017, during Trump's first year in office, his administration raised the idea of separating children from their parents as a way to deter illegal immigration -- a key plank of his campaign.

Officially launched in April 2018, the "zero-tolerance" policy allowed criminal proceedings to be brought against anyone who crossed the US-Mexico border illegally, resulting in parents being immediately taken into custody without their children.

Lawsuits and a public outcry, even among his own Republican Party, forced the Trump administration by June 2018 to halt the separations.

In practice, however, the Trump administration continued to separate families under another regulation which allowed undocumented parents to be arrested and deported if they had committed a serious crime.

According to NGOs, the rule was inappropriately applied to minor offenses, particularly traffic violations.