Honda motorsport boss Yasuhisa Arai thinks will would be "very difficult" for the Japanese manufacturer to supply Toro Rosso with engines in 2016.
Red Bull and junior team Toro Rosso are currently without a power unit supply for next season after splitting with Renault. Red Bull is in negotiations with Ferrari after Mercedes -- who will now supply Manor in 2016 -- ruled out an engine supply.
However, talks with Ferrari are ongoing and Red Bull boss Christian Horner admitted time is "more critical" for Toro Rosso than it is for the senior outfit. A Honda engine supply is the only other option for Toro Rosso and would make it easier for Red Bull to secure a Ferrari deal as it would be one less team for the Italian firm to supply, as F1 debutants Haas will join Sauber as Maranello customers next season.
Honda only has an engine deal with McLaren, a partnership which has struggled so far in 2015, and Arai says it needs to work on rectify its current problems before taking on another team.
"It is difficult for [Toro Rosso] to come to us," Arai told F1i. "It is difficult, looking at the maturity of the power unit it is very difficult. We have to put ourselves in the right place first."
Toro Rosso boss Franz Tost did not rule out Honda as a power unit supplier but said an uncompetitve season would be detrimental to the team from a commercial standpoint.
"First we must look that we have a competitive car because we rely on sponsors and we have a responsibility to show good performance for our partners," Tost said. "What the outcome will be, which engine we will use next year we will see soon. We must decide soon because otherwise time is running away from a design point of view."