He also suggested, in an apparent reference to the clergy sex abuse scandal, that seminary training needs to be tightened to instill a commitment to "holiness and spiritual wisdom."
The pope has raised the sex abuse scandal and other problems facing the U.S. church as American bishops have been making a periodic visit to the Vatican throughout the year.
In Friday's address to bishops from Iowa, Kansas, Nebraska and Missouri, the pope outlined how bishops must provide for the future of the church.
"No one can deny that the decline in priestly vocations represents a stark challenge for the church in the United States, and one that cannot be ignored or put off," the pope said.
He urged a program of vocational promotion and a national day of prayer for priestly vocations.
The U.S. Conference of Catholic Bishops says nearly 500 new priests were ordained in 2003, down about half from 1965.
The pope's reference to seminaries comes amid plans for a Vatican-sponsored investigation of U.S. seminaries, a project stemming from the abuse crisis. The outgoing president of the U.S. Catholic bishops, Bishop Wilton D. Gregory, has said onsite visits should start within a year.