Two top U.S. law firms are in advanced merger discussions to create one of the 10 largest in the country, according to the firms' chairmen.

San Francisco-based Orrick Herrington & Sutcliffe is expected to sign a letter of intent next week to join with Pillsbury Winthrop Shaw Pittman, a source with direct knowledge of the discussions said.

Pillsbury Chairman James Rishwain and Orrick Chairman Mitch Zuklie issued a joint statement to Reuters that said: "Our firms are in exploratory discussions about a possible combination. These talks are serving to confirm the great respect our firms have for each other."

(Reporting by Casey Sullivan; Editing by Kenneth Barry)