The 38-year-old actress, known for her humanitarian work, said the kidnappings were "an unthinkable cruelty."
"Sadly, of course, there is real evil in the world," she continued. "You watch the news and you see all of the people suffering and so much cruelty."
On Monday a man asserting to be Boko Haram leader Abubakar Shekau claimed responsibility for the kidnappings and vowed to sell the girls.
"I abducted your girls. I will sell them in the market, by Allah," the man said. "There is a market for selling humans. Allah says I should sell. He commands me to sell. I will sell women. I sell women."
Eight more schoolgirls were reportedly abducted Tuesday from Warabe, a village in northeastern Nigeria. The kidnapping comes three weeks after 276 girls were taken away on trucks from a boarding school in the town of Chibok late in the evening.
U.S. Secretary of State John Kerry called the abductions "an unconscionable crime" on Saturday and said that America "will do everything possible to support the Nigerian government to return these young women to their homes and to hold the perpetrators to justice."