WASHINGTON — The Trump administration said Thursday that it would start blocking a small number of asylum seekers from entering the United States from Mexico, using the San Ysidro border crossing near San Diego as the first location to turn back immigrants applying for refugee status.

The policy to block asylum seekers was first announced last month by Kirstjen Nielsen, the homeland security secretary, and Secretary of State Mike Pompeo. It will gradually be expanded over the next two weeks at border crossings with heavy foot traffic in Texas, New Mexico and Arizona, according to a senior United States official briefed on the move, who spoke on the condition of anonymity.

The move is intended to dissuade immigrants, mostly from Central America, from making the long and dangerous journey through Mexico to the southwestern United States border. The policy is likely to intensify pressure on the Mexican authorities, who are already struggling to deal with thousands of Central American immigrants who have applied for humanitarian visas in Tapachula, near the border with Guatemala.