Vin Diesel has branded Fast & Furious 7 the "hardest film I've ever made" following the death of his longtime co-star Paul Walker last year (13).

Production on the seventh installment of the blockbuster franchise was halted following the actor's death in a car crash November (13), and his brothers Caleb and Cody stepped in to shoot remaining scenes for their sibling.

Fast & Furious 7 recently wrapped filming, and Diesel tells Entertainment Tonight the latter part of production was the most difficult experience he has ever had on a movie set.

He says, "There's only one Paul Walker. When you've worked with someone for 15 years and you know each other's idiosyncrasies and you've created magic together for so long, it's a strange thing to mourn that person and simultaneously pretend that person is next to you.

"Fast 7 was the hardest movie I've ever had to make and in some ways the most important movie I've ever had to make."

However, Diesel praised the Walker brothers for lending a hand in the most unfortunate of circumstances, adding, "It was validating in a way that his family cared so much about the saga and the continuation and the completion of Paul's last work, but it didn't make the process any easier."

Fast & Furious 7 hits cinemas on 3 April (15).