Kubica has been driving for Renault at in-season testing

Renault say there are "still question marks" over whether Robert Kubica can make a full-time return to Formula 1 after life-changing injuries.

The 32-year-old Pole has had three tests with Renault in the last three months, six years after he suffered a partially severed right arm in a rally accident.

Renault F1 boss Cyril Abiteboul told BBC Sport: "Have we definitively answered the question whether he can race again? No."

Kubica's manager said he believed he had "shown he can be a serious candidate".

Alessandro Alunni Bravi said Kubica's goal was to race for Renault in 2018, adding: "Our goal was to become a potential candidate for Renault for 2018. The goal remains to come back as a race driver but it is not our decision. It's not in our power."

Kubica completed 142 laps - two grand prix distances - in temperatures in the mid-30Cs in his test in a Renault 2017 car at the Hungaroring on 2 August.

He set the fourth fastest lap on the day and was 0.157secs slower than Renault's second driver Jolyon Palmer managed in qualifying at the Hungarian Grand Prix four days beforehand. The Englishman was 0.8secs off the pace of team-mate Nico Hulkenberg in qualifying.

Archive: Kubica's remarkable return to Formula 1

It was his first run in a contemporary F1 car since early 2011. A matter of days after setting the fastest time for Renault in the first pre-season F1 test, Kubica crashed during a rally in northern Italy and suffered multiple injuries, from which he has faced a long rehabilitation.

Abiteboul said: "We have some indication, some confirmation he is a fantastic professional, that he is extremely talented, that he has a huge hunger for racing and to make a return to professional racing whether in F1 or somewhere else.

"But whether he can race in F1, there is more work to be done."

Will Kubica test for Renault again?

Abiteboul said he was not able to say whether Renault would be able to give Kubica further tests to fully answer their concerns over his ability to return as a fully fledged F1 race driver.

"We would like to," he said, adding: "We will try but there is limitation. It is going to be difficult in the timing and framework we have."

This is a reference to the fact that testing is severely restricted in F1 - the only chance to run Kubica before a post-season test in Abu Dhabi would be in a practice session at a grand prix weekend.

Cyril Abiteboul made a return to Renault Sport F1 as a managing director after two years with Caterham F1 Team

Abiteboul added that this may not fit in with Renault's desire to finalise their 2018 driver line-up before the end of the season.

He said: "Having said that, we have not done all of this in order to stop at the first difficulty. We knew there would be difficulty. Whether we can go further or not is still a little bit up in the air but we will see in the next two to three weeks what the future can offer to the team and Robert."

Where does Kubica stand?

Kubica chasing 'impossible' F1 dream

Bravi said: "We are really thankful to Renault for the opportunity and just waiting to understand if there can be any further opportunities.

"For sure Robert's priority is to work with Renault because he feels part of this family.

"After the test, he said he was not 100% happy - that was because he missed an opportunity with ultra-soft tyres to do his optimum lap time because of a red flag and his feeling was that he made progress each time he went out on track.

"So he feels there is a lot of room for improvement. His potential to improve is bigger than the others' because he has been out for seven years.

"This is a starting point. We are not putting any pressure on Renault. He did his job. The decision is up to Renault."

Sainz a hot tip for Renault?

Nico Hulkenberg is sixth in the drivers' standings and Jolyon Palmer 19th

Nico Hulkenberg is contracted for next season but team-mate Jolyon Palmer is expected to be dropped after a disappointing season, even though Abiteboul has said the Englishman still has a chance to convince the team to keep him.

Renault are said by sources to have Toro Rosso driver Carlos Sainz at the top of their list for 2018.

The Spaniard is under contract to Red Bull but the team have made it clear he would be available to Renault at a price. Initial negotiations have taken place but no deal has yet been struck.

Abiteboul said: "We had an interest in Sainz starting actually last year. We continue to have an interest because I think he is doing a good season.

"But he has a contractual situation which Red Bull may or may not be willing to discuss. We will see. Things have been fairly quiet over the summer shutdown and I expect the next couple of weeks are going to be very intense on that topic."

Other drivers linked to Renault next year include Force India's Sergio Perez and McLaren's Fernando Alonso.

Bravi said that if Renault decided not to pursues their interest in Kubica, he had "other opportunities", both in F1 and other categories of motorsport.