Red Bull will drop drivers who don't perform, Marko warns

"We basically have long-term contracts at the Red Bull junior programme and the whole programme is based on performance," Red Bull advisor Marko told the official F1 website.

"So no shoot-out, but be very clear: the one who is not delivering goes," he added.

Red Bull Racing has Daniel Ricciardo and Daniil Kvyat under contract, while Toro Rosso drivers Max Verstappen and Carlos Sainz are entering their second year at the junior team.

Verstappen, who impressed in his rookie season after becoming the youngest ever driver to compete in a race, said recently that his priority was to "move forward" in the near future.

Kvyat came under pressure early in the 2015 season after struggling to match his teammate, but the Russian went on to enjoy a strong second half of the year and finished ahead of Ricciardo in the standings.

When asked if he could see Verstappen racing for Red Bull next year, Marko said: "Let's see what he delivers. The second season is sometimes trickier than the first."

Red Bull future still unclear

Red Bull will run Renault engines again this season after failing to secure a new deal with either Mercedes or Ferrari.

The team spent a big part of 2015 threatening to leave F1 if it could not find a competitive engine that would allow it to fight for victories.

Although in the end Red Bull confirmed its presence on the grid this year, Marko insisted that having a deal to stay in the sport until 2020 did not mean much unless it gets a competitive power unit soon.

"The Concorde Agreement goes until 2020. But to stay that long we must have a competitive engine in the very near future," he said.

Marko said Red Bull's goal for this year is to finish third, but he conceded that it was not enough for the team in the long term.

"We will try to fight Williams. I think that at the end of the season we should be third. But, of course, in the long run that is not good enough."