Sauber is not talking to any manufacturers other than Ferrari about a 2014 engine supply, but admits it has not secured a deal for Formula 1's new technical era.
With some details of next year's turbocharged V6 engines still to be finalised, Sauber team boss Monisha Kaltenborn said her team was in limbo regarding its future power source.
"It's a very interesting situation for customer teams at the moment. There is so much, as we understand, that is not clear at the moment, so it's very difficult to engage into these kinds of talks," she said.
"For us, we have had a partnership with Ferrari for more than 10 years, so that's the natural link for us to go.
"But they also have difficulties, because nothing has been clarified down to the last detail.
"It's extremely challenging, and a fear we have as a private team is to make sure we don't have any disadvantage compared to the manufacturer teams.
"It comes down to finding the engine, knowing the technical details of it and knowing what impact it has on our design."
She dismissed suggestions that Sauber might be looking at a switch to Mercedes engines.
"Just to be very clear on this, we are not talking to anyone else, unlike what was reported," Kaltenborn insisted.