Illegal immigration across the southwestern U.S. border increased 15 percent in July over the previous month, the Department of Homeland Security said Tuesday.

"The short answer is we don't know what all of those causes are," DHS spokesman David Lapan told reporters, particularly in light of claims by President Donald Trump that illegal immigration at the U.S. border with Mexico has plummeted since his inauguration.

The DHS data was reported by The Washington Times.

Just last month, Trump claimed that illegal immigration was down 78 percent since January.

"The border is down 78 percent," Trump told law enforcement officers in New York last month. "Under past administrations, the border didn't go down — it went up.

"But if it went down 1 percent, it was like this was a great thing. Down 78 percent.

"The southern border of Mexico, we did them a big favor. Believe me.

"They get very little traffic in there anymore, because they know they're not going to get through the border to the United States," Trump said.

According to the agency, Border Patrol agents arrested 13 percent more illegals in July versus the previous month — while officers at various ports of entry saw a 23 percent increase.

July usually experiences a drop, the Times noted, so the increase signaled a change in general seasonal patterns.

In addition, the number of arrests of families and unaccompanied alien children also gained in the month.

Arrests of minors rose 27 percent, while family apprehensions rose 46 percent.

Regarding ports of entry, Border Patrol officers arrested 6,833 aliens trying to enter the U.S. in July — and 18,198 illegals who snuck across the border.

The total, 25,031, while the highest under Trump, remains lower than any other month under former President Barack Obama since December 2011, the Times reports.