* comScore counter sues Nielsen over patent violation
* Nielsen sued comScore earlier for patent infringement
* Both sides seeking unspecified damages (Adds Nielsen comment)
NEW YORK, March 22 (Reuters) - Two of the biggest online measurement companies are locked in a battle over alleged patent infringement.
ComScore SCOR.O filed a counter suit against Nielsen Holdings (NLSN.N) 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.
"While we support vigorous competition in the marketplace, that competition must be fair and respect intellectual property rights," Nielsen said in a statement. "Nielsen does not take such matters lightly and only after thorough analysis and consideration did we file this action. We look forward to a fair and appropriate resolution."
A representative from comScore was not immediately available for comment.
In the government filing, comScore said it "intends to vigorously protect its intellectual property and defend itself."
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 (DIS.N) ESPN.com, Yahoo YHOO.O and Google (GOOG.O).
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 and Carol Bishopric)