The Catholic Diocese of Brownsville said Sister Leticia Ugboaja was detained by agents as she was walking to attend mass at Our Lady of Sorrows Church in Texas.
Pope Leo XIV prayed for political leaders to end the "fratricidal hatred" in the world, as he spent the anniversary of his election in the ancient city of Pompei.
The Vatican says the "need to work tirelessly in favor of peace" was discussed in talks with U.S. Secretary of State Marco Rubio.
Pope Leo XIV paid tribute to Pope Francis on Tuesday on the first anniversary of his death, recalling his preaching about God's mercy and his gestures of solidarity with poor people.
Pope Leo XIV blasted the "handful of tyrants" who are ravaging Earth with war and exploitation, as he preached a message of peace.
President Trump has criticized Pope Leo for his stance on the Iran war.