Spirit Airlines is advising its shareholders to reject a takeover bid for JetBlue Airways shares three days after it turned hostile in its bid to create what would be the country’s fifth-largest airline.

Spirit reiterated on Thursday that any attempt to merge with the New York carrier would face significant regulatory hurdles, largely due to JetBlue’s alliance with American Airlines in the Northeast. The Justice Department is suing to block that deal.

JetBlue is “in the middle of a merger with American Airlines, one of the big three they claim to compete with, and then tries to buy a competitor and take seats out of the market and raise fares, and that going to be a big deal that our board felt was insurmountable,” Spirit CEO Edward Christie told CNBC.

JetBlue responded Thursday, saying Spirit’s potential deal with Frontier Airlines was also under regulatory review. JetBlue tried to woo these Spirit investors, saying Frontier offers less value, more risk and no regulatory commitment.

“Both agreements are subject to regulatory review, and both agreements have a similar risk profile,” the company said in a statement. “Spirit shareholders recognize this and are very interested in learning more about our superior offering and the regulatory commitments and protections we have made, including a reverse severance fee.”

JetBlue has offered to buy Spirit Airlines after a proposed acquisition of that carrier by Frontier Airlines, a deal that Spirit backs despite a lower bid price.

On Monday, JetBlue launched a hostile takeover bid for Spirit, directly asking shareholders of the low-cost carrier to vote against a tie-up with Frontier Group Holdings Inc. of Colorado.

JetBlue’s Monday offer was $30 per share in cash, or more than $3.2 billion, but said its April 5 offer of $33 per share is still available if Spirit enters negotiations. Spirit’s board of directors rejected JetBlue’s initial $3.6 billion offer on May 2.

Shareholders of Miramar, Fla.-based Spirit are due to vote June 10 on Frontier’s cash and stock offer, worth about $2.9 billion when it was announced in February. .

Shares of Spirit fell 2.6% before the market opened, while shares of JetBlue fell slightly.