MEMPHIS, Tenn. (AP) — FedEx Corp. says it will buy up to 30 Boeing aircraft from United Airlines and convert them into cargo planes.

Financial terms of the deal were not released.

FedEx disclosed its plans in a regulatory filing. The package-delivery company said it would buy 14 Boeing 757 aircraft, which will be delivered through 2015, and could buy up to 16 more 757s.

United announced last year that it would buy 150 Boeing 737s over the next several years to replace older 757s.

FedEx spokesman Jess Bunn said Monday that the purchase of planes from United is the latest move in a fleet-modernization program that started several years ago.

"We are retiring older, less efficient aircraft and replacing them with more modern and efficient ones," he said.

Shares of FedEx fell $1.46 to close at $97.02. Shares of United Continental Holdings Inc. rose 3 cents to $31.90.