The battle to buy a stake in Japan Airlines Corp. intensified Thursday, when AMR Corp.'s American Airlines said it was prepared to invest $1.1 billion in the ailing Japanese carrier along with private-equity firm TPG, trumping a previous offer from rival Delta Air Lines Inc.
American and Delta are openly wrangling over the unprecedented chance to invest in JAL, Asia's leading carrier by revenue and a former prized national asset. In a country where negotiations are traditionally handled discreetly and behind closed doors,...