– President Donald Trump said Thursday that he wants to send 2,000 to 4,000 National Guard members to the U.S.-Mexico border to help federal officials fight illegal immigration and drug trafficking.

Trump’s comments to reporters on Air Force One were his first estimate on guard levels that he believes are needed for border protection. It would be a lower number of troops than the 6,400 National Guard members that former President George Bush sent to the border between 2006 and 2008.

Trump said his administration is looking into the cost of sending the troops to the border and added “we’ll probably keep them or a large portion of them until the wall is built.”

Earlier Thursday, Ronald Vitiello, the U.S. Customs and Border Protection’s acting deputy commissioner, cautioned against a rushed deployment. “We are going to do it as quickly as we can do it safely,” he told Fox News Channel.

He said that guard members would be placed in jobs that do not require law enforcement work, an apparent reference to undertaking patrols and making arrests.

The National Guard in Texas expressed support, but said that deployment remained in “very early planning stages.” The Republican governors of New Mexico and Arizona have also backed the deployment.

At the Pentagon, Marine Lt. Gen. Kenneth F. McKenzie said that it has not yet been determined how many, if any, of the troops in the border security operation will be armed.

With troops in all states, the National Guard has been called on by past presidents and governors to help secure U.S. borders. Trump ordered the deployment because “we are at a crisis point” with illegal immigration, said Secretary Kirstjen Nielsen of the Department of Homeland Security.

Nielsen said guard members would provide support to border officials, “help look at the technology, the surveillance, in some cases we’ll ask for some fleet mechanics” and free up agents trained in law enforcement for other duties.

Arrests along the border with Mexico jumped to 50,308 in March, a 37 percent increase from February, and more than triple the same period last year. Border arrests rose 10 of the past 11 months after falling in April 2017 to the lowest since the Department of Homeland Security was created in 2003.

But Mayor Dee Margo of El Paso, Texas, told NPR he was not convinced extra forces are needed for his border city, which he called “the safest” in the U.S. “We already have a fence that was established during the Bush administration that runs through the city,” he said. Instead of more troops, “what I would love to see is a better understanding of what truly goes on on the border.”