Red Bull says Renault's concept for its 2016 has potential, but admits there are still doubts over the French manufacturer's ability to deliver the improvements.

After approaching Mercedes, Ferrari and Honda for engines in 2016 and being turned down by all three, Red Bull finally settled for another year with Renault . As part of a new sponsorship deal the engines will run under the TAG Heuer brand, but Red Bull will receive the same specification of power unit as the returning Renault works team.

Renault struggled for power and reliability throughout the 2015 season and its performance update at the penultimate race failed to deliver the step in power it had hoped for. However, Red Bull advisor Helmut Marko says Renault should be able to deliver an improvement next year.

"Renault has presented concepts that look good," Marko told Speedweek. "Now these need to be implemented and deliver in practice what has been promised in theory. The question is how quickly it will be possible."

Red Bull approached engine specialist Ilmor this year to help Renault with the development of its internal combustion engine. Ultimately the Ilmor concept did not run on track, but the company is due to continue to work with Renault next year.

"We will have the same engines as the new Renault works team," Marko confirmed. "Mario Illien will cooperate with his Ilmor company as a consultant with Renault to help develop the power unit."

Red Bull's deal with Renault is just for one year, but Marko said it could be extended if the French manufacturer starts to deliver an improvement in performance.

"We hope to secure an alternative engine from 2017, but if the Renault power unit develops well during the 2016 we will consider extending the contract."