JetBlue Airways Corp. on Monday improved its offer to buy smaller rival Spirit Airlines Inc , as the battle for the low-cost carrier intensifies.

Under the improved proposal, JetBlue offers Spirit's shareholders $31.50 per share in cash, which is comprised of $30 per share at deal close and the prepayment of $1.50 per share of the reverse break-up fee.

The $1.50 per share will be paid as a cash dividend if Spirit shareholders vote for the JetBlue-Spirit deal.

JetBlue's earlier offer was for $30 per share.

Shares of Spirit Airlines Inc jumped 6.2% to $22 premarket.

JetBlue also raised the reverse break-up fee by $150 million to $350 million, which is payable to Spirit shareholders in case the deal falls through due to regulatory reasons.

The improved proposal comes days after Frontier Group Holdings Inc agreed to pay a break-up fee of $250 million in a bid to salvage its $2.9 billion acquisition of Spirit.