SAN JOSE, Calif. — Malware warnings halted Internet users from visiting popular sites across the Internet today after the Web site of an Internet advertising company was hacked. The company said today that its ads were not infected with any virus, so other sites were safe.

According to Twitter users, sites such as the New York Times, the Huffington Post, Los Angeles Times, Washington Post, and many others were being blocked by Google’s Chrome browser with warnings about possible malware — malicious software that could infect a user’s computer — emanating from ad company Netseer.

“Content from cm.netseer.com, a known malware distributor, has been inserted into this Web page. Visiting this page now is very likely to infect your computer with malware,” a Chrome message said today when a user attempted to visit the San Jose Mercury News’ Web site.

Netseer, a Santa Clara, Calif.-based startup founded in 2006 that helps target ads based on Web site content, suffered a hacking attack on its Web site but said today that it was not actively issuing malware-infected ads.

“This morning at approximately 5:30 a.m. Pacific time, our third-party hosted corporate Web site (netseer.com) was hacked and infected with malware. Consequently, Google added our domain to the list of malware affected Web site. Our operations team went into all-hands-on-deck mode, and we have successfully cleaned the site of the malware issue,” spokesman Kathleen Formidoni said.

Because the company’s corporate site and ad-serving infrastructure share the same domain, the block Google served to keep malware placed on Netseer’s Web site from spreading also affected ads Netseer placed on other sites. But “the malware was never served into ad serving stream,” Ms. Formidoni wrote in an email.

In an email response to a request for comment, a Google spokesman said the company does not comment on individual malware cases.

First Published February 5, 2013, 12:21 p.m.