Mastercard revealed on Sunday that it will investigate allegations against Pornhub.com after a New York Times column reported that videos on the adult website involved the abuse of children.
The NTY article, written by Nicholas Kristof, outlined that certain videos include assault on unconscious women and girls, according to the author.
"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 on Friday.
Mastercard told Reuters that it was launching an inquiry into the allegations with the banking partner for Pornhub's parent company, Luxembourg-headquartered MindGeek.
"If the claims are substantiated, we will take immediate action", the banking provider said.
Billionaire investor Bill Ackman urged Mastercard and Visa to temporarily drop payments to Pornhub after the column was published.
In the tweet on Saturday, Ackman asked American Express to take action, despite that Amex is not usable on adult sites.
The Pershing Square founder also called for porn sites to review all videos before posting, and that age and consent of all those involved must be confirmed.
Pornhub.com adamantly denied the allegations and issued a statement in response, saying that any "assertion that we allow CSAM (child sexual abuse material) is irresponsible and flagrantly untrue", Reuters reports.
The adult site observed that it has a team of human moderators who manually review "every single upload", alongside automated detection technologies.
Leading politicians, including Canadian Prime Minister Justin Trudeau, responded to the NYT column, saying that his government was working with law enforcement and security officials to address the complaints.
US Republican Senator Josh Hawley said he would introduce legislation that would make it a federal right for people to sue if they have been "coerced or trafficked or exploited" into participating in adult videos that are uploaded to and hosted by sites like Pornhub.
In November, money transfer service Paypal released a statement saying that it would stop providing support for Pornhub.com after the adult entertainment site made certain payments to employees and models without permission.