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.

Frontier Airlines is making another offer to merge with bankrupt Spirit Airlines, less than two years after a previous plan fell through.

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