Spirit Airlines said in regulatory filings Wednesday that it is rejecting the latest merger offer from Frontier Airlines, and will instead continue with its bankruptcy restructuring plan.

Frontier (ULCC) announced Wednesday morning that it had made a "compelling proposal" to merge with Spirit, and said it believed the deal would provide more value to Spirit's creditors and shareholders than the discount airline's current bankruptcy plan.

In separate filings with the Securities and Exchange Commission, Spirit and Frontier on Wednesday each laid out communications that have taken place over the last several weeks following a renewed offer from Frontier. The communications were made public through the filings Wednesday after non-disclosure agreements expired Wednesday morning.

Spirit Says Latest Frontier Offer Has 'Many Deficiencies'

In the latest letter dated Jan. 28, Spirit executives told Frontier that its proposal still has "many deficiencies" after the weeks of discussions, and said they would be pursuing their solo bankruptcy plan. The discount airline said it expects to exit bankruptcy in the first quarter of this year.

"While we appreciate your continued interest and share your view of the logic of a potential transaction, your January 7 terms (which have not been improved on in the last three weeks) are both inadequate and unactionable," Spirit executives said in the letter. "In light of the above, our Board has directed management and advisors to proceed with confirmation of our extremely efficient standalone reorganization that will position us well for the future."

Spirit has fielded a variety of takeover offers from Frontier and JetBlue (JBLU) over the last several years, with legal challenges blocking the JetBlue merger and Frontier eventually abandoning the negotiations in November, shortly before Spirit filed for bankruptcy.

Frontier shares were up about 4% Wednesday morning.