Japan Airlines Corp. is leaning toward maintaining its longstanding alliance with AMR Corp.’s American Airlines instead of teaming up with Delta Air Lines Inc., according to a person familiar with the matter.

American’s bid to keep JAL has gained traction recently at least in part because of growing concerns a JAL-Delta partnership would trigger antitrust concerns in the U.S., added people close to the protracted tug-of-war for Asia’s largest carrier.

Sticking with American would represent a reversal from just a few weeks ago, when the Japanese government was in favor of a joint venture with Delta, which leapfrogged American to become the largest U.S. carrier in 2008 after merging with Northwest Airlines.

No final decision has been made, according to people familiar with the matter. An official decision on a partner is slated to be made this week or early next week, according to Masaru Onishi, the new president of JAL.

JAL’s new chairman, Kazuo Inamori, started from scratch in deciding whether Asia’s biggest carrier by revenue should ally itself with Fort Worth, Texas-based American or Atlanta-based Delta in a joint venture on trans-Pacific flights, according to one person close to the deliberations.

Mr. Inamori, who took over as JAL’s chairman Feb. 1, has had conversations with officials in Washington focusing on whether a Delta-JAL tie-up would receive antitrust immunity, this person added.