Fernando Alonso is not sure he will be able to win next month's Indy 500.

The Spaniard, who quit F1 altogether at the end of last year, almost won the fabled event in 2017 as he pushes for the 'triple crown'.

He tested his car, operated by McLaren and painted in the orange and blue F1 livery, in Texas this week.

Alonso is quoted by Marca sports newspaper as saying the new aerodynamic kit has less downforce that he remembers, making the car "trickier to drive".

"I guess it's going to be challenging, especially this year as I am not with the Andretti environment with their support. It's all by ourselves this year," he said.

Indeed, McLaren boss Zak Brown wanted to go it alone for Alonso's 2019 bid, putting former Force India boss Bob Fernley in charge of the Indy 500 team.

"I think it is quite important to set up everything as a team," Alonso said.

"Who does what in the team, also the pit wall, the facility is completely new for everyone and to try to solve as many problems here and at the test."

But he said he is still targeting victory.

"Absolutely, that's the goal," said Alonso. "After winning Le Mans, only the 500 is missing."

Asked if he will come back again if he fails to win the race in 2019, Alonso answered: "Well, it's a possibility, but I hope we will be competitive this year too."

And he is also not ruling out a potential run at the full Indycar season.

"It was something we considered last year but the season for me was very intense with the full WEC schedule and Daytona. It is not something to rule out for the future but right now no," said Alonso.