The United States will return the first group of migrants seeking asylum in the United States to the Mexican border city of Tijuana on Friday, a spokesman for Mexico's president said on Thursday.

In a major policy change, U.S. President Donald Trump's administration said on Dec. 20 it would send non-Mexican migrants who cross the U.S. southern border back to wait in Mexico while their U.S. asylum requests are processed.

The spokesman for President Andres Manuel Lopez Obrador's office did not specify the nationalities of those to be returned to Mexico, although the policy was aimed at helping cope with rising numbers of Central Americans seeking asylum in the United States.

The two countries have held two meetings to work out details of the plan to return migrants seeking U.S. asylum across the shared border. Mexico has said it will not accept anybody facing a credible threat in Mexican territory.

Serious doubts exist over whether Mexico can keep Central American asylum seekers who are fleeing poverty and crime safe, especially in border towns that are often more violent than the cities they left.

It is unclear how Mexico plans to house what could be thousands of asylum seekers for the months — or years — it takes U.S. immigration cases to be heard. A backlog of more than 800,000 cases is pending in immigration courts.