Mastercard Inc said on Sunday it was investigating allegations against Pornhub.com following a newspaper column which said many videos posted on the adult website depicted child abuse.

The New York Times column, written by Nicholas Kristof, described videos on Pornhub that the author said were recordings of assaults on unconscious women and girls.

"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. 

Pornhub denied the allegations.

"Any assertion that we allow CSAM (child sexual abuse material) is irresponsible and flagrantly untrue," it said in a statement emailed to Reuters.

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.

Visa said it is aware of the allegations and is "actively engaging with the relevant financial institutions to investigate," while also engaging directly with MindGeek.

"If the site is identified as not complying with applicable laws or the financial institutions' acceptable use policies and underwriting standards they will no longer be able to accept Visa payments," the company said in a statement.

American Express said it has a longstanding global policy that prohibits acceptance of its cards on digital adult content websites.

Ackman suggested it should be made illegal for porn sites to post videos before they are reviewed by a monitor, and until the ages and consent of participants have been validated.

In its response, Pornhub said it has a vast team of human moderators who manually review "every single upload," as well as automated detection technologies. It did not say how many people were part of its review team.

Kristof's column also drew reactions from politicians including Canadian Prime Minister Justin Trudeau, who said his government was working with police and security officials to address the issues it raised.

In the United States, Senator Josh Hawley said he will introduce legislation to create a federal right to sue for every person "coerced or trafficked or exploited by sites like Pornhub."