There have been several close calls. On two occasions, Russian aircraft flew within 1,500 feet and 500 feet of American warplanes, although not in a threatening manner. Such incidents underscore the need for such an agreement, Mr. Cook said.

“The Russians need to abide by these flight safety protocols that they’ve now agreed to, because we don’t want miscalculation and misunderstanding,” he said.

The protocols call for aircraft to maintain a “safe distance” from one another, Mr. Cook said, refusing to elaborate on what that distance is but saying coalition aircrews know how close is too close. “There’s no need for them to have an encounter if everyone’s abiding by these rules,” he said.

The text of the memo was not released. Mr. Cook said the Russians had requested that it not be shared. He declined to elaborate on the ground communication line, including where exactly it would be located, but described it as a backup resource to “have real-time conversations” in case something went wrong with the air communications.