Alitalia has opened its books to the three groups looking to buy a stake in the airline but the sale has been clouded by worries over its finances.
European airlines are going through a period of restructuring
Aeroflot, the Russian airline which is among those vying for a 39% stake in Alitalia, said that the Italian firm's position could be worse than was known.
Alitalia made a loss of 626m euros (£423m) last year amid fierce industry competition and high costs.
The sale is conditional on Alitalia retaining its identity and key routes.
Aeroflot, which has teamed up with Italian bank Unicredit, is competing against a consortium led by private equity firm Texas Pacific, and a group including rival airline Air One.
While Aeroflot said that it was committed to help turn around Alitalia's fortunes, it expressed concern about the firm's financial situation.
"I suspect that the figures for Alitalia could be even worse than expected," said Aeroflot's deputy chief executive Mikhail Poluboyarinov.
The share sale is expected to be completed by the end of July.