Mastercard is investigating allegations against Pornhub.com, after a New York Times column on Friday accused the porn site of hosting videos on its platform depicted child abuse.

“The issue is not pornography but rape. Let’s agree that promoting assaults on children or on anyone without consent is unconscionable,” Kristof wrote in the column published on Friday. (nyti.ms/2JXiqSw)

The credit card company, which accepts payments on the website, said on Sunday it has “zero tolerance for illegal activity” on its network and that it works closely with law enforcement and organizations such as the National and International Center for Missing and Exploited Children to detect and prevent those types of transactions.

Mastercard told Reuters in a statement that it was investigating the allegations with Pornhub’s parent MindGeek’s bank. “If the claims are substantiated, we will take immediate action,” Mastercard said.

Billionaire investor Bill Ackman called on Mastercard and Visa Inc to temporarily withhold payments to Pornhub following the newspaper column. Ackman also asked American Express Co to take action, though the company’s cards aren’t accepted on the site.

Pornhub denied the allegations, stating that any assertion it allows child sex abuse material (CSAM) on its platform is “irresponsible and flagrantly untrue.”

“Due to the nature of our industry, people’s preconceived notions of Pornhub’s values and processes often differ from reality — but it is counterproductive to ignore the facts regarding a subject as serious as CSAM, ” Pornhub said in a statement to CNN Business. “We have zero tolerance for CSAM. Pornhub is unequivocally committed to combating CSAM, and has instituted an industry-leading trust and safety policy to identify and eradicate illegal material from our community.”

Pornhub said it has a vast team of human moderators who manually review “every single upload,” as well as automated detection technologies.