Lufthansa sees no limit to the number of planes and crews it could lease from Air Berlin, its chief executive told German newspaper Bild am Sonntag, amid criticism that support for its ailing rival is a stealth takeover attempt.

"We already support Air Berlin, in that we have leased 38 planes and set them on our routes. I can imagine, however, that we would expand this cooperation and lease further Air Berlin planes and crew," Carsten Spohr said.

"For me, there is no upper limit to this. On the other hand, I do not currently envisage a takeover of the company."

Spohr has previously expressed interest in loss-making Air Berlin on condition its debt pile and costs could be reduced.

Travel agencies, the German monopoly regulator and rival carrier Ryanair have raised competition concerns over any possible takeover of Air Berlin, Germany's second-biggest airline, by flagship Lufthansa.

Air Berlin, 29 per cent-owned by Abu Dhabi-based carrier Etihad, is trying to protect roughly 8,000 jobs in Germany. Last year, the carrier made a record net loss of €782 million.