The number of illegal immigrants apprehended at the southwest border in 2017 hit a 45-year low, but the Department of Homeland Security warned the numbers cannot continue to decrease without changes to long-standing immigration policy.

Border Patrol reported a 40 percent decrease in apprehensions in 2017 under President Trump compared to 2016, when former President Barack Obama was in office.

"The final border apprehension numbers of 2017, specifically at the southern border, undeniably prove the effectiveness of President Trump’s commitment to securing our borders," acting DHS press secretary Tyler Houlton said in a statement. "The secretary will require fixes to these loopholes as part of any immigration package negotiated today at the White House."

DHS said 40,513 people were taken into custody by CBP officers in December. The number was up 1,500 from November and nearly 6,000 higher than October, the first month of the 2018 fiscal year.

But in December 2016, CBP said apprehended nearly 30 percent more illegal immigrants: 58,412.

During her meeting with lawmakers and Trump at the White House on Tuesday, DHS Secretary Kirstjen Nielsen stressed the need to reform existing policy.

"The significant increase over the last month in the number of family units and unaccompanied children coming across the border illegally highlights the dire need for Congress to immediately adopt responsible pro-American immigration reforms," Houlton said. "Current loopholes in our immigration laws have created an incentive for illegal immigrants who knowingly exploit these same loopholes to take advantage of our generosity."