Jean-Eric Vergne says Toro Rosso has been held back by Renault's power-unit so far in testing.

Toro Rosso completed just 139 laps across the four days of the second test in Bahrain, with only Red Bull, Lotus and Marussia managing a lower total. On the final day Vergne was restricted to 19 laps due to a problem with the Renault engine, leaving the team on the back foot heading in to the final test this week.

Asked by if it was a case of Renault holding Toro Rosso back so far, Vergne replied: "Yes.

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"Sometimes it's the engine and sometimes it's ourselves. But the problem is when you stay in the garage so much. You see the problems with a new car when you drive, so that's a bit of an issue."

But Vergne said there was still progress being made by both Renault and Toro Rosso and that he was hopeful of a swift recovery.

"I will not talk negatively because I fell like it and everybody knows it, so I don't need to say anything else. I'm positive. We still have major problems and they were able to fix some of them. I know it's going to take time, I knew arriving here that nothing would have been fixed [immediately].

"I have big faith in the people working in the team and also in the Renault people. They all know what the problems are and I am confident they can do something about it."