Migrants who want to request asylum have camped out on the international bridge leading into Brownsville, Texas, causing a closure of the span.
Dozens of migrants from Central America and elsewhere stretched out on the bridge before dawn, with some sleeping.
A Mexican official who was not authorized to be named confirmed the blockage of the bridge leading out of Matamoros, Mexico.
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The official said the migrants were tired of waiting to make their initial claims for asylum at a U.S. border crossing.
Under a policy know as metering, U.S. officials at many border bridges accept only a few asylum-seekers per day. The Associated Press found about 19,000 names on waiting lists in four border cities visited in late July.