Red Bull, which currently uses Renault engines, has been using Toro Rosso to monitor Honda's operation and progress after the junior team switched from a Renault supply for this season.

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 is the first formal conversation between Honda and Red Bull about a potential supply and the conditions each party would require 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 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 Grand Prix, 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 2010-2013, but has endured a fractious relationship since F1 introduced V6 turbo-hybrids that soured so much Red Bull entered a sponsorship deal with Tag Heuer to brand its engines.

Red Bull has won just nine races since the start of 2014 as Mercedes and Ferrari have outgunned Renault in F1's new engine era.

Daniel Ricciardo's victory in China this year means Red Bull has 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 is ready to gamble on Honda, which would be its first de facto works engine supplier since Renault returned with a race team for 2016.

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 is what triggered the dissolution of the McLaren-Honda union in the first place.

McLaren finally lost patience during the third season of poor results with Honda, which has struggled to make up ground after returning to F1 in the second season of the current engine regulations.

It thus triggered a complicated swap deal to ditch Honda for Renault, with Toro Rosso going the other way.