A deal price could not be immediately obtained, but it will be financed partially in cash and partially in stock, the sources said.
Shares of Pinnacle were up 2.51 percent on CNBC's report, giving it a market capitalization of $8.10 billion, while shares of Conagra were down 0.49 percent, giving it a market capitalization $15.25 billion.
The sources, who requested anonymity because the information is confidential, cautioned that a deal could still fall apart. Conagra and Pinnacle both declined to comment, citing policies against discussing market speculation.
A pairing of Healthy Choice-owner Conagra and Bird's Eye-owner Pinnacle would combine two companies with a large presence in frozen foods at a time when the category is seeing a resurgence. It would create the second-largest U.S. frozen food company behind Nestle, analysts at RBC Capital Markets have written.
It would also be the culmination of on-again, off-again talks the two have had for years. It comes months after activist investor Jana Partners disclosed a roughly 9 percent stake in Pinnacle and said it planned to talk with the company about a possible sale.
Jana has a track record with the pairing, having previously taken a stake in and pushed for changes at Conagra.
Meantime, Conagra CEO Sean Connolly brings to the deal his own track record. Connolly was CEO of Hillshire Brands when it attempted to buy Pinnacle in 2014. Hillshire, though, ultimately scrapped that deal in favor of a sale to Tyson Foods.