A large number of migrants in Mexico who were "posing a potential threat to make a mass entry" into the United States have been prevented from crossing a bridge leading to El Paso, Texas, a US Customs and Border Protection spokesman says.

Physical barriers were put up to restrict entry at the Paso Del Norte International Bridge on Sunday afternoon, spokesman Roger Maier said in a statement.

Barricades also were used for a short time on Sunday afternoon at an El Paso crossing at the Bridge of Americas and at one in Stanton, Maier said.

Traffic was reopened and flowing in both directions as of Sunday evening, he said.

Maier said the Office of Field Operations Special Response Team and US Border Patrol agents assisted customs officers.

It was not immediately known what caused the attempted mass crossing.