WASHINGTON — The Trump administration said on Friday that it could immediately fund 100 miles of new and replacement border fencing, a first step in the president’s plans for building a wall on part of the United States’ nearly 2,000-mile border with Mexico.

The replacement fencing includes 28 miles in the San Diego region and a new 30-foot-high barrier in Calexico, Calif., that extends for two miles. In New Mexico, the Border Patrol will replace 20 miles of barriers that are intended to stop vehicles with new fencing to prevent people from crossing into the United States.

Ronald D. Vitiello, the acting deputy commissioner of Customs and Border Protection, described the construction as part of a new wall system because it would replace older, outdated barriers. Customs and Border Protection is the parent agency of the Border Patrol.

A mix of barriers — like chain-link fences, steel walling and beams — already stretches across more than 650 miles of the border. Ultimately, Mr. Vitiello said, the administration aims to cover 1,000 miles on the border with fencing or a wall.