Spiralling fuel costs and rising competition have rattled Delta

The comments came in an internal memo from chief executive Gerald Grinstein, dated 26 July, reports said. Delta was unavailable for comment.

Analysts said the memo reinforced fears Delta could be heading for bankruptcy.

The news comes days after the firm said it had cut its losses to $388m in the second quarter against $1.97bn in 2004.

Shares in the group fell by as much as 20% on Wall Street before ending the day 11.8% lower at $2.99.

Debt worries

The news was not enough to prevent Mr Grinstein warning that the firm could still be in for some turbulent times.

"In light of what we have accomplished together so far, there can be no doubt that Delta's transformation plan is delivering results. What is also clear is that they are not enough," his memo said.

He added that the cost of interest on the firm's massive debts and rising fuel bills meant further cuts were needed.

Earlier this year the number three US carrier warned it may run out of money as it battled tough conditions.

In response the group has tried to cut costs by agreeing a new pay deal with pilots.

Earlier this month it also raised ticket prices in a bid to offset rising fuel bills.

The Atlanta-based group has also held talks with a group of lenders, led by General Electric and American Express, about restructuring its debts.