NEW YORK – AT&T Inc and Time Warner Inc's AOL unit have agreed to purge their Internet servers of Web sites that traffic child pornography, New York state's attorney general said Thursday.

The pacts with two of the largest U.S. Internet service providers (ISPs) follow similar agreements last month with Verizon Communications Inc, Sprint and Time Warner Cable.

The companies have also agreed to shut down access to child pornography newsgroups, online bulletin boards that are a major supplier of illegal images, New York Attorney General Andrew Cuomo said. The agreements will affect Web users throughout the country, not just in New York.

“These agreements with two of the nation's largest ISPs to eradicate child porn Web sites from their servers tighten the noose around this despicable trade,” Cuomo said in a statement.

He said that as part of the investigation, his office developed a new system for identifying online content that contains child pornography. He said that every online picture has a unique “hash value” that once identified and collected, can be used to digitally match the same image elsewhere.

By building a library of these hash values, investigators were able to filter through tens of thousands of online files at a time, quickly identifying which ISPs were providing access to child pornography images.

AT&T said it was “happy to work with Attorney General Cuomo's office and the National Center for Missing and Exploited Children in the effort to help prevent the distribution of this harmful and abusive content,” according to a statement from David Condit, the company's president of state legislative and regulatory affairs.

Ira Parker, executive vice president and general counsel at AOL, said the company “is proud to support Attorney General Cuomo's efforts to fight online child pornography.”

(Editing by Derek Caney)