san diego - The number of Venezuelans, Cubans and Nicaraguans taken into custody at the U.S. border with Mexico soared in August as migrants from Mexico and traditional sending countries were stopped less frequently, authorities said Monday.

U.S. authorities stopped migrants from Venezuela, Cuba and Nicaragua about 56,000 times last month, up from 49,826 times in July and 23,141 times in August 2021, according to administration officials. At the same time, fewer migrants were stopped from Mexico and the Central American "Northern Triangle" countries of Guatemala, El Salvador and Honduras for a third straight month.

Overall, migrants were stopped about 203,000 times. They were stopped 199,976 times on the U.S. border with Mexico in July and 213,593 times in August 2021.

The growing numbers from Venezuela, Cuba and Nicaragua, are the latest sign of rapidly changing migration flows as U.S. authorities wrestle with unusually large influxes.

While no single reason can be pinpointed, it is extremely challenging for the U.S. to expel migrants from those countries under a pandemic-era rule known as Title 42, which U.S. officials invoke to deny a chance at seeking asylum on grounds of preventing the spread of COVID-19. U.S. relations with all three countries are deeply strained, making it difficult to impossible to send them home.

Mexico agrees to accept migrants expelled under Title 42 if they are from Guatemala, Honduras or El Salvador, in addition to Mexico. While the rule applies to all nationalities in theory, people from those four countries are most affected.

The Biden administration has leaned on other countries in the Americas to absorb more people fleeing their homes, including Mexico, Costa Rica, which is home to many Nicaraguans, and Colombia, which has taken in millions of Venezuelans in recent years.

Venezuelan migration plummeted early this year after Mexico introduced restrictions on air travel but has increased in recent months as more come over land through the notoriously dangerous Darien Gap in Panama.

In July, Venezuelans were stopped 17,651 times on the U.S. border with Mexico, most of them in and around Eagle Pass and Del Rio, Texas.

Reminders of their presence are in daily headlines. The roughly 50 migrants that Florida Gov. Ron DeSantis flew to the upscale Massachusetts island of Martha's Vineyard were all Venezuelan, as were five of the six bodies that U.S. authorities found drowned to death in the Rio Grande near Eagle Pass on Labor Day weekend. The sixth was from Nicaragua.