By Ted Hesson

WASHINGTON (Reuters) -U.S. authorities caught more than 171,000 migrants at the U.S. border with Mexico in March, according to preliminary data shared with Reuters, the highest monthly total in two decades and the latest sign of the mounting humanitarian challenge confronting President Joe Biden.

The preliminary March arrest totals at the U.S.-Mexico border represent the highest monthly level since April 2000, when border patrol agents caught more than 180,000 migrants.

The total includes about 19,000 unaccompanied migrant children and 53,000 family members traveling together, the figures show. Single adults made up roughly 99,000 of the total.

The Biden administration is struggling to house newly arrived unaccompanied children, who are exempted from expulsion under a COVID-19 health order known as Title 42. Children have been backed up in crowded border stations and processing centers for days.

The shelter system that houses the children has been overwhelmed and U.S. officials have scrambled in recent weeks to open emergency shelters, including sites in convention centers in Dallas and San Diego.

Central American and Mexican migrants have made up the bulk of arrivals in recent months, in keeping with trends in recent years.

The March figures show a 178% increase in the number of migrant families caught at the border compared with last month.

While Biden said last week that the “vast majority” of families are being sent back to Mexico under Title 42, U.S. government data suggests that is not the case.

More than half of the 19,000 family members caught at the border in February were not expelled, according to public U.S. Customs and Border Protection (CBP) data, with many released into the United States to pursue immigration court cases.

Reuters also obtained three daily U.S. border enforcement reports in March that showed only 14-16% of family members were expelled on those days.

A CBP spokesman said official statistics would likely be released next week and declined to comment further.

U.S. Department of Homeland Security (DHS) spokeswoman Sarah Peck said last week that given fluctuating migration flows, “one day or week of statistics doesn’t reflect the full picture.”

Peck said the department’s policy is still to expel families “and in situations where expulsion is not possible due to Mexico’s inability to receive the families, they are placed into removal proceedings.”

U.S. border agents have encountered more repeat crossers in the past year compared with recent years.

(Reporting by Ted Hesson; Additional reporting by Kristina Cooke in San Francisco; Editing by Howard Goller)