|Russian Grand Prix on the BBC|
|Venue: Sochi Dates: 9-11 October|
|Coverage: Live coverage on BBC TV and BBC Radio 5 live, online, mobile, the BBC Sport app and Connected TV. Full details here|
Red Bull are in talks with Formula 1 boss Bernie Ecclestone to try to solve a deepening engine supply crisis that could lead them to quit the sport.
The team are finalising a split from engine partner Renault but have been unable to secure a replacement on terms that satisfy them.
Mercedes have refused to supply them and Ferrari will not sell them their definitive 2016 engines next year.
Ferrari are offering 2015 engines but Red Bull want parity of performance.
Owner Dietrich Mateschitz has said he will pull the team out of the sport if they do not get it.
But Ferrari's position is that at this late stage they do not have the logistical or technical capacity to give 2016 engines to Red Bull in addition to their other two customers, Sauber and the new Haas team.
F1 insiders have raised the possibility of Red Bull carrying on with Renault despite everything - their existing deal has not been completely terminated and it runs to the end of 2017.
But the two parties have fallen out following the collapse of trust on both sides, and reviving their relationship would be difficult.
Renault are also in the final stages of completing a takeover of the Lotus team and returning as a full factory entrant.
Red Bull team principal Christian Horner and motorsport adviser Helmut Marko held a long meeting with Ecclestone on Friday morning at the Russian Grand Prix to discuss the situation.
Ecclestone then immediately went to talk to Mercedes, continuing his push to persuade the world champions to supply Red Bull.
Horner said: "There are lots of discussions going on, but nothing is concluded, nothing is confirmed.
"Bernie has influence. He is a promoter, he wants Red Bull to be there - all the teams, Lotus included. He has been very helpful in trying to navigate a way through these issues."
Ecclestone said the situation was "sorted" but Horner suggested that was not the case when he commented: "The great thing about Bernie is he seems to know a lot more than the rest of us. If he knows it's all sorted, that's all fine then."
Mercedes F1 boss Toto Wolff has made it clear the company is not interested in supplying Red Bull.
Wolff told BBC Sport: "The situation hasn't changed - we have decided not to pursue that option."
And he ruled out supplying a 2015-spec engine to Red Bull next season, saying Mercedes were "not set up" to do so.
If Red Bull were to pull out of F1, both the senior team and junior outfit Toro Rosso would leave.
Ferrari's Sebastian Vettel, who won his four world titles with Red Bull, said a withdrawal would be "a huge shock".
"Red Bull has been part of F1 for 10 years and has supported it ever since," the German said.
"Of course, I have strong links to Red Bull and I know the team like the back of my hand. This is why I struggle to imagine them to pull out. I hope they will stay in the sport."