The Sun-Times Media Group, owner of the Chicago Sun-Times and dozens of suburban newspapers, filed for Chapter 11 bankruptcy Tuesday, making it the fifth newspaper publisher to seek protection from creditors in recent months.

The step, brought on by a precipitous decline in advertising revenue, means both of Chicago's major daily newspapers are operating under bankruptcy protection. Tribune Co., the parent company of the Chicago Tribune, Los Angeles Times and other newspapers, filed for Chapter 11 in December.

The Sun-Times Media Group, which filed in a Delaware court, said it will continue to operate its print and online properties. The company listed $479 million in assets and $801 million in debt. The largest unsecured creditors are newsprint vendors. Three are owed more than $1 million each.

The company has retained Rothschild Inc. to help with a possible sale of assets.

"We firmly believe that filing for Chapter 11 protection and exploring the potential sale of assets or new investment in the company offers us the best opportunity to protect our respected media properties for the long-term," Jeremy Halbreich, the company's interim chief executive, said in a statement.

The Sun-Times, the company's flagship newspaper, had a paid weekday circulation of about 313,000 as of September, ranking it 17th in the U.S.

The dire financial condition of Chicago's newspapers mirrors the situation in Philadelphia, where the publisher of The Philadelphia Inquirer and Philadelphia Daily News filed for bankruptcy protection in February.

Other cities with two daily newspapers have seen the industry's crisis whittle away competition this year. The Rocky Mountain News closed, leaving The Denver Post, while the Seattle Post-Intelligencer went online only, leaving The Seattle Times without a mainstream daily print rival.