President Donald Trump has for months pushed his administration officials to re-instate its policy of separating migrant families at the US-Mexico border NBC News reported Monday.

Homeland Security Secretary Kirstjen Nielsen, who resigned on Sunday , was reportedly opposed to re-implementing the deeply controversial "zero-tolerance" policy.

Trump signed an executive order in June 2018 reversing his family separation policy following widespread outrage, including among Republicans.

President Donald Trump has for months pushed his administration officials to re-instate its policy of separating migrant families at the US-Mexico border, NBC News reported on Monday morning, citing three US officials.

Homeland Security Secretary Kirstjen Nielsen, who resigned on Sunday, was reportedly opposed to re-implementing the deeply controversial "zero-tolerance" policy, citing last year's federal court rulings and Trump's own June 2018 executive order reversing the family separation policy.

"We're going to keep families together but we still have to maintain toughness or our country will be overrun by people, by crime, by all of the things that we don't stand for and that we don't want," Trump said during his announcement of the executive order last year.

A source told NBC News that Trump believes separating thousands of migrant children — including infants and toddlers — from their parents was successful in deterring immigrants from crossing the southern border.

Read more: Kirstjen Nielsen was reportedly kicked out of the Trump administration by tweet, the latest in a long line of public firings by the president

Government officials told a federal court last week that it could take up to two years to identify many of the children who were separated under the policy last year and are no longer in government custody. And government inspectors announced that thousands more migrant children may have been separated from their families by the Trump administration beginning in 2017.

Nielsen's sudden departure is reportedly part of a larger effort by Stephen Miller, a top White House adviser and immigration hardliner, to overhaul DHS policy and implement a harsher crackdown on immigration, according to CBS News.

Michelle Mark contributed to this report.