President Vladimir V. Putin said Thursday that he would sign into a law a ban on adoptions of Russian children by American citizens, retaliating against an American law that punishes Russians accused of violating human rights and dealing a potentially grave setback to bilateral relations…

Pavel Astakhov, Russia’s child rights commissioner and a major proponent of the ban, told news agencies on Wednesday that he expected it to be enacted and to immediately block the departure of 46 children ready to be adopted by parents from the United States. He said the adoptions would be blocked regardless of previous agreements with the United States and even though some of the adoptions had already received court approval, and he expressed no regrets over the likely emotional turmoil for the families involved.

“The children who have been chosen by foreign American parents — we know of 46 children who were seen, whose paperwork was processed, who came in the sights of American agencies,” Mr. Astakhov said in his statement. “They will not be able to go to America, to those who wanted to see them as their adopted children. There is no need to go out and make a tragedy out of it.”