Red Bull's short-list to partner Sebastian Vettel next season is Lotus's Kimi Raikkonen plus Toro Rosso pair Daniel Ricciardo and Jean-Eric Vergne.
The second seat is available following Mark Webber's decision to quit Formula 1 at the end of the season.
Red Bull team boss Christian Horner said it would be "foolish to ignore" Raikkonen but that Ricciardo and Vergne were "true contenders".
Lotus team principal Eric Boullier said he hoped Raikkonen would stay.
"Red Bull is chasing Kimi and we want to keep Kimi," the Frenchman said. "He will decide what he wants to do."
Boullier said Lotus had already been in talks with Raikkonen about next season and that the Finn had been seeking clarity about the future of the team following a change of ownership structure in the last two weeks.
Owners Genii Capital has sold a 35% stake to a US-Middle Eastern investment consortium called Infinity Racing.
Boullier said: "He knows what he has with us. It is attractive to go to Red Bull but he knows what he has and now we are talking with him and he is seeking some understanding of where we are going because in terms of ownership we are new. So far he is happy with what he has and we will see.
"The environment suits him. He can see every year we keep improving the team performance and we expect to keep going this way and match Christian's team's performance as soon as possible."
Horner said: "We will take our time, we don't need to be in a rush. We will evaluate our options and come to a decision when we can.
"Any driver is going to want to be in the most competitive environment he can be in."
Frenchman Vergne and Australian Ricciardo are the latest members of the Red Bull junior driver programme to drive for Toro Rosso as part of the company's assessment programme.
So far, only one driver - Vettel - has graduated from Toro Rosso since the team made their debut in 2006.