July 10 (Reuters) - Bristol-Myers Squibb Co said on Thursday it plans to seek U.S. marketing approval in the third quarter for its experimental cancer immunotherapy Opdivo as a treatment for advanced melanoma, which would be months sooner than Wall Street expected.

The company said it would seek approval to market the drug to patients who had previously been treated with Bristol-Myers' already approved melanoma drug Yervoy, which works through a different mechanism than Opdivo.

Mark Schoenebaum, an analyst with ISI Group, said many analysts had not expected Bristol-Myers to submit the marketing application until mid-2015, although he and some others had speculated an earlier filing was possible.

The U.S. drugmaker in April, in a so-called rolling submission that will be completed by the year end, asked the U.S. Food and Drug Administration to approve Opdivo (nivolumab) to treat patients with another type of cancer, squamous cell lung cancer.

Opdivo works by blocking a protein called PD-1, thereby enabling the immune system to recognize cancer cells and then attack them. It is considered one of Bristol-Myers' most important experimental drugs, with potential to generate multibillion-dollar annual sales, if approved.

Bristol-Myers shares were little changed in premarket trading.

(Reporting by Ransdell Pierson; Editing by Meredith Mazzilli)