People walk past the building of Los Angeles Times newspaper, which is owned by Tribune Publishing Co, in Los Angeles, April 27, 2016. REUTERS/Lucy Nicholson

Gannett Co Inc (GCI.N), the publisher of USA Today, raised its unsolicited offer for Tribune Publishing Co (TPUB.N) to $15 per share from $12.25, valuing the publisher of the Chicago Tribune and the Los Angeles Times at about $475 million.

Including debt, the offer is worth about $864 million. Tribune's shares rose 22.1 percent to $14 shortly after midday, below the latest offer, which was set at a premium of 31 percent to Tribune's Friday close.

Tribune's board adopted a shareholder rights plan earlier this month to thwart Gannett.

Gannett said the latest bid followed an analysis of Tribune's debt and pension liabilities.

"In addition, after further review, Gannett has greater confidence in its ability to yield additional operational improvements in this transaction," the company said.

Tribune said it would thoroughly review the revised offer.

Oaktree Capital Group LLC (OAK.N), the third-largest shareholder in Tribune, urged the company to negotiate a deal with Gannett earlier this month. Oaktree holds a 14.8 percent stake in Tribune.

The offer comes as newspapers struggle with declining circulation, rising costs and falling advertising sales, pushing publishers to consolidate and find new areas of growth.

Gannett shares were 1.6 percent higher at $15.88.