Arrests for illegal border crossings rose for the third month in a row in May according to statistics released Wednesday by the Department of Homeland Security (DHS).

The hike shows more people are seeking to illegally cross the border despite President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE's efforts to crack down on illegal immigration, the signature issue of his presidency.

The rising number of arrests has frustrated the president, who criticized his Homeland Security secretary at a Cabinet meeting this spring. Trump has also been frustrated with Congress's inability so far to fund a wall on the Mexican border — a structure the president said Mexico would pay for during his campaign.

Customs and Border Patrol statistics show that a more than 2,000-strong surge of arrests of unaccompanied minor children at the southern border contributed to May being the highest month so far for arrests of immigrants at the border in fiscal 2018.

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Total arrests along the U.S.-Mexico border topped 40,000 last month, according to the agency, which is used as a benchmark for understanding the level of illegal immigration occurring at the border.

A spokesman for DHS said the rising numbers show a "sustained effort" will be necessary to curtail illegal crossings.

"These numbers show that while the Trump administration is restoring the rule of law, it will take a sustained effort and continuous commitment of resources over many months to disrupt cartels, smugglers, and nefarious actors," spokesman Tyler Houlton said in a statement.

"No one expects to reverse years of political inaction overnight or in a month," he said. "It is also clear change will take more than Administration action alone. Congress must act to end legal loopholes that have left us with policies that serve as tremendous magnets for illegal immigration."

Houlton also blamed the refusal by members of Congress "to close catch-and-release loopholes" for preventing the administration from controlling the border.

The administration has taken to separating parents from their children if they are caught illegally crossing the border, and has also stepped up enforcement within the United States, including a raid Tuesday at a gardening company in Ohio that saw more than 100 arrests.

Another raid earlier this year, during which the agency arrested dozens of undocumented immigrants at a Tennessee meatpacking plant, was Immigration and Customs Enforcement's largest workplace raid in a decade.

The crackdown has seemed to have little effect on the overall numbers of immigrants crossing the U.S.-Mexico border illegally, as the arrest numbers have been steadily climbing since January, though some of the rise can be attributed to regular seasonal workers.

An expanding economy in the United States has lowered the unemployment rate to 3.8 percent. But this could also be a factor in the rising border crossings as people seek work in the United States.

The rise in illegal immigration has frustrated the president, who reportedly sharply criticized Homeland Security Secretary Kirstjen Nielsen Kirstjen Michele NielsenMore than million in DHS contracts awarded to firm of acting secretary's wife: report DHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections MORE at a Cabinet meeting earlier this year over the rising numbers.

On Tuesday, Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE defended the Trump administration's policy of separating families detained at the border, accusing immigrants of "recklessly" crossing with their children in tow.

"If people don’t want to be separated from their children, they should not bring them with them. We’ve got to get this message out," Sessions said.

"And you can’t be giving immunity to people who bring children with them recklessly and improperly and illegally," he continued. "You have to — you will be prosecuted if you bring, if you come illegally. And if you bring children, you’ll still be prosecuted."