$6.66 billion in May fuel spending marked the second straight month above $6 billion for U.S. airlines, after April's $6.47 billion.
$4.09 per gallon average jet fuel prices drove the surge, up 85% from $2.21 a year earlier, while consumption slipped 0.6% to 1.627 billion gallons.
Middle East conflict and disrupted Strait of Hormuz shipping pushed energy costs sharply higher this spring, prompting airlines globally to raise fares, add fees and trim schedules.
$2.88 per gallon at major U.S. hubs on Tuesday shows costs have eased since mid-June after a U.S.-Iran interim ceasefire, though tanker strikes and tighter U.S. curbs on Iranian oil keep the truce fragile.
Delta reports second-quarter results Friday, with investors expected to focus on whether the recent fuel-price pullback can ease pressure on airline finances.