Frontier Airlines has sweetened its offer to combine with fellow budget carrier Spirit, less than a week before Spirit shareholders are set to vote on the deal.

The new offer for $4.13 per share, $2 per share higher than Frontier's original cash-and-stock bid, comes after JetBlue Airways repeatedly upped its own offer to buy Spirit outright in an all-cash deal.

Spirit shareholders are set to vote on the Frontier deal on Thursday.

"We will more thoroughly review and assess the revised terms of the Frontier-Spirit merger agreement, and we intend to continue our 'vote no' campaign against the inferior Frontier transaction at the special meeting," JetBlue said in a statement Friday.