WASHINGTON — When the number of National Guard troops stationed at the Mexican border is reduced next year, their duties are expected to shift from being on the border to flying above it.

About 1,200 troops now help Border Patrol agents spot illegal border crossers and human and drug smugglers. The office of Rep. Michael McCaul said Wednesday the contingent is expected to be cut by about half. The Texas Republican opposes reducing the guard force at the border.

A federal official briefed on the new strategy said the reduced force will focus its efforts on aerial surveillance. The official spoke on the condition of anonymity because the plan has not been announced.

Defense Dept. spokesman Lt. Col. Robert L. Ditchey said officials are still working out details of the future of Guard troops at the border.