WASHINGTON, Aug 16 (Reuters) - The administration of U.S. President Joe Biden on Monday appealed a federal judge"s order to reinstate a controversial immigration program that forced tens of thousands of migrants to wait in Mexico for the resolution of their U.S. asylum cases. The Biden administration appealed the decision to the conservative-leaning 5th U.S. Circuit Court of Appeals after a Texas-based judge on Friday vacated the administration"s decision to end the program, known as the Migrant Protection Protocols (MPP). The MPP program was launched under former President Donald Trump in 2019, triggering objections from immigrant rights groups and Democrats who said it forced migrants to wait in dangerous conditions in Mexico and undermined the ability to seek asylum in the United States. The Biden administration halted the program in February and has since allowed about 13,000 migrants enrolled in it to enter the United States to pursue their cases. Biden"s decision to terminate the program - informally known as "remain in Mexico" - was an early and significant political victory, energizing his supporters and underscoring his stated commitment to a more humane immigration system. Republicans criticized the move, saying ending MPP and other restrictive Trump policies would encourage more illegal immigration. In the months that followed, the number of migrants caught crossing the U.S.-Mexico border soared to 20-year highs, a development that was immediately seized upon by Republicans. read more The states of Texas and Missouri filed a legal challenge in April over Biden"s decision to wind down the MPP program, saying it was "unexplained and inexplicable." U.S. District Court Judge Matthew Kacsmaryk said on Friday that the Biden administration had failed to follow proper regulatory procedure when it ended the program and ordered it restarted. read more However, Kacsmaryk, a Trump appointee, stayed his decision for a week to allow the Biden administration to appeal. Reporting by Ted Hesson in Washington; Editing by Ross Colvin and Aurora Ellis