Germany's Lufthansa Group has formally submitted a proposal to acquire parts of Air Berlin to the struggling carrier's administrator.
Lufthansa says it has presented a term sheet – a document outlining conditions for investment – which confirms its interest in Air Berlin.
Submission of its proposal means Lufthansa has "firmed up" its interest with a broader overall concept, the flag-carrier states.
Lufthansa had previously confirmed it would enter negotiations over a possible acquisition of Air Berlin assets.
But while it has solidified its interest, Lufthansa Group has not disclosed specific details, adding that the content of the letter of intent remains confidential.