Pope Francis heaped praise on Catholic nuns in his first New York appearance, calling them "women of strength," in a move to salve wounds opened by his predecessor who had launched a probe into what he had regarded as radical feminists.

A six-year clash between the Vatican and the Leadership Conference of Women Religious ended in April, after the group, which represents about 80 percent of U.S. nuns, agreed to demands they keep within Roman Catholic doctrine.

"I would like to express my esteem and gratitude to the religious women of the United States," Francis said in an evening prayer service at New York's St. Patrick's Cathedral, one of the highest-profile churches in the United States.

"What would the church be without you?" said Francis, who succeeded Pope Benedict in 2013. "Women of strength, fighters ... I wish to say 'thank you,' a big 'thank you.'"

Benedict had launched a probe into some 341 religious orders representing 50,000 nuns in 2008, after criticizing nuns as taking a soft line on issues such as the use of birth control and homosexual activity, both of which the church opposes.

The probe attracted public attention in the United States, with many Catholics complaining that the male-dominated Vatican was unfairly picking on nuns who have played a huge role in church education and hospitals over the past few decades.