WASHINGTON, June 25 (Reuters) - U.S. authorities have made more than 1 million arrests of migrants at the U.S.-Mexico border so far in fiscal year 2021, according to preliminary figures shared with Reuters, a tally that underscores the immigration challenges facing President Joe Biden. At the current pace, the total border arrests for the fiscal year, which ends on Sept. 30, would be the highest since 2000, when nearly 1.7 million migrants were apprehended by U.S. authorities. Biden, a Democrat who took office five months ago, has reversed many of the hardline immigration policies put in place by his Republican predecessor, former President Donald Trump. Republicans blame Biden"s policies for the upsurge in illegal border crossings in recent months, but migration experts say poverty, violence and food insecurity are factors driving migrants to leave Guatemala, Honduras and El Salvador. U.S. Border Patrol made 172,000 migrant arrests at the southwestern border in May, on par with 20-year highs from March and April. Similar figures are expected in June. The current demographics of migrants arriving at the border, including many from Central America and other countries, take longer to process than the mostly Mexican men who arrived at the border in 2000, according to a U.S. Customs and Border Protection official who spoke to Reuters on condition of anonymity.