YUMA, Ariz. (AP) — Trump administration officials on Friday touted the 100th mile (161st kilometer) of border wall to be built since the president took office as crossings have continued to drop.

Acting Secretary of Homeland Security Chad Wolf responded to critics who say the new construction is only replacing old fencing. The new, 30-foot (9 meter) walls do replace other barriers, but those were shorter and easier to cross. Wolf said he's confident the administration will build or start to build 400 to 450 miles (644 to to 724 kilometers) by the end of the year.

“The wall system you see behind me is an undeniable impediment to smugglers, traffickers and other criminals who have exploited our lack of effective border infrastructure to smuggle drugs, illicit goods and engage in human trafficking,” Wolf said.

He spoke in Yuma, Arizona, a far-flung region of the Southwest near California that's seen dramatic highs and lows in illegal border crossings over the past two years. Most of those crossing are families.

Officials in Yuma are building a 5-mile (8-kilometer) section of wall along the Colorado River, where the Border Patrol says most migrants crossed.

Yuma's nearly 97% drop in the number of families since May is partially because the area's adoption of the Remain in Mexico program, which forces asylum-seekers to wait south of the border while their cases wind through court.

On Monday, the Yuma Sector began implementing a program first tested in El Paso, Texas, that fast-tracks asylum claims but is being challenged in federal court. The Prompt Asylum Case Review requires asylum-seekers to wait in Customs and Border Protection custody while their cases are decided within 10 days. Immigrant rights activists say it puts migrants, especially children, at risk by keeping them in facilities where allegations of abuse and mistreatment run rampant.

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Associated Press reporter Astrid Galván in Phoenix contributed to this report.