(Photo credit: REUTERS/DAVID MDZINARISHVILI)
Red Bull, which uses Renault engines, have been using Toro Rosso to monitor Honda's operation after the junior team switched from a Renault supply for the 2018 F1 season.
Dr Helmut Marko, Red Bull's motorsport advisor, met with Honda's motorsport boss Masashi Yamamoto in the build-up to this weekend's Azerbaijan Grand Prix.
The Baku meeting was the first formal conversation between Honda and Red Bull and about a potential link-up, and gave each party a chance to outline the conditions that would be required to make a deal happen.
Red Bull and Honda appear to be happy to work to the May 15 deadline for confirming an engine supply outlined in the FIA's F1 sporting regulations.
There is flexibility in this date, provided all the engine manufacturers and the FIA agree, but Renault has indicated it will need to stick to the deadline if it is to continue supplying Red Bull beyond its current agreement, which ends this year.
Renault is understood to be planning a significant performance step for June's Canadian GP, which could coincide with the next major upgrade from Honda.
This would present a key comparison between the two manufacturers' performance and development levels if Red Bull has not firmed up an engine supply deal by then.
The Red Bull-Renault alliance won four world titles from 2010 to 2013, but has endured a fractious relationship since F1 introduced V6 turbo-hybrids.
Red Bull has won just nine races since the start of 2014 as Mercedes and Ferrari have out-gunned Renault in F1's new engine era.
Daniel Ricciardo's victory in China this year meant Red Bull won a race before Mercedes for the first time since 2013, but the start of his season has also featured a couple of high-profile failures.
This has heightened speculation Red Bull are ready to gamble on Honda, which would be its first de facto works engine supplier since Renault revived its factory team programme, and Red Bull rebadged its customer Renaults as Tag Heuers.
Honda is still seeking to improve its energy recovery systems and its internal combustion engine technology and is yet to prove it is ready to produce a race-winning engine, but there is evidence of more progress in 2018.
The best McLaren-Honda result across three years was fifth place, which Toro Rosso beat on only its second start with Honda this season when Pierre Gasly finished fourth in Bahrain.
Doubt over Honda's short-term capabilities triggered the dissolution of the McLaren-Honda union. McLaren finally lost patience during the third season of poor results with Honda, which has struggled to make up ground since returning to F1 in the second season of the current engine regulations. This triggered a complicated swap deal in which McLaren switched to Renault, with Toro Rosso going the other way.
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