Spirit Airlines rejected a $400 million merger proposal from Frontier this week, saying the offer was financially inadequate and came too late. Spirit Airlines declined an opportunity to merge with rival low-cost airline Frontier on Wednesday.

Spirit, which filed for bankruptcy protection last year, says it has rejected the new Frontier overture but would be open to a higher offer.

The Denver-based low-cost carrier said its combination with the Dania Beach, Florida-based Spirit would create a stronger low-cost carrier and benefit “from the very significant synergies Frontier expects to achieve by combining the airlines’ operations.”