Two of the biggest online measurement firms are locked in a battle over alleged patent infringement.

ComScore filed a counter suit against Nielsen Holdings and its Netratings division in a United States District Court alleging infringement of patent rights, comScore revealed in a regulatory filing on on Tuesday.

Nielsen is claiming that comScore violated its patent rights and filed a lawsuit on March 16 in a U.S. District Court.

Both companies are seeking unspecified damages and injunctive relief.

Representatives from comScore and Nielsen were not immediately available for comment.

ComScore and Nielsen's Netratings are the dominant online measurement sources that track the number of people and the amount of time they spend on some of the world's most popular websites such as Walt Disney's, Yahoo and Google.

Their findings, considered the currency of Internet advertising, are used to set ad rates based on the number of people visiting a website.

(Reporting by Jennifer Saba; Editing by Gary Hill)