DENVER (AP) — Frontier Airlines is sweetening its offer for Spirit Airlines just a week before Spirit shareholders vote on the deal.

Spirit and Frontier said late Thursday that they have amended their agreement, and Frontier will pay a $250 million termination fee if it can’t complete the deal.

JetBlue, which is also trying to buy Spirit, has already offered a $200 million breakup fee.

Shareholders of Florida-based Spirit are scheduled to vote June 10 on Frontier’s stock and cash offer.

JetBlue is asking Spirit shareholders to reject the Frontier bid and tell the Spirit board to negotiate with JetBlue.