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.

Budget carrier Frontier Airlines announced on Wednesday that it has made another bid for fellow discount carrier Spirit Airlines, which is in bankruptcy, for an undisclosed sum.