The Swiss team's chief headed into the last two races of this season declaring that, with Marcus Ericsson now under contract for 2017, the Swede's teammate would be named before the chequered flag in Abu Dhabi.
However, Kaltenborn has now told Swiss newspaper Blick that the second driver question is "still wide open".
Asked if reports were true that Felipe Nasr, having lost the bulk of his Banco do Brasil sponsorship, will be replaced by Pascal Wehrlein, she explained: "Felipe Nasr is still possible, but we talk with quite different drivers from different continents.
"We would like to clarify the driver question as soon as possible."
Sauber finished 2016 in 10th place with two points.