Alain Prost has conceded that top three teams Mercedes, Ferrari and Red Bull are "unreachable" for Renault this year.

The F1 legend is an advisor to the French works team, which had targeted getting closer to the top teams in 2019.

"You have to be realistic, and the three top teams are unreachable," Prost told Canal Plus.

"It's all about trying to get closer."

Renault made a big step with its engine in 2019, but the biggest problem at present is reliability.

"We must keep developing but also work on reliability," Prost said.

"I think we will still have one or two races that are a little difficult, but after that it will start to get better," he added.

Referring to the top two teams Mercedes and Ferrari, he said they are "war machines".

"They have more resources and a lot more personnel than us, so we have to be more ingenious, but it's true that the rules do not really allow for ingenuity," said Prost.

"We have improved the car, but they do as well and maybe even better than us. We know that we have to move up a gear.

"There are a lot of things coming, so let's see if we can recover the gap. There is no big problem with the programme we have, except that we are a little less good in terms of performance but especially reliability early this season," he added.

"On the other hand, in engine performance, I can tell you that we are not bad at all."