Aston Martin's Fernando Alonso will be a threat to Red Bull's unbeaten run for the Formula One season in Sunday's Monaco Grand Prix, according to last year's winner Sergio Perez.

Champions Red Bull have won all five races so far but Monaco's tight and twisty street circuit — the slowest on the calendar — gives rivals a fighting chance.

Alonso, whose last win was a decade ago with Ferrari, has finished third in four races this season and is the best of the rest after Red Bull's championship leader Max Verstappen and Perez. He is also a two-time winner in Monaco and feels his car is well suited to the challenges of the principality, while the Red Bull enjoys less of a speed advantage.

"We certainly know that it’s not our strongest circuit," Perez told reporters. "But it's Monaco and anything can happen.

"Fernando," added the Mexican when asked who posed the biggest threat.