McLaren driver Fernando Alonso believes Mercedes look "stronger than ever" after seeing their form in the first two days of Formula One testing. 

Despite Ferrari's Sebastian Vettel topping the timesheets on both days, the Mercedes drivers completed far and away the most laps out of any other competitor.

Reigning world champion Lewis Hamilton clocked 156 laps around the Circuit de Barcelona-Catalunya in the first test on Monday, before team-mate Nico Rosberg completed 172 on day two. 

And Alonso, a two-time world champion, feels that kind of performance from Mercedes shows just how strong they can be again this season.

"I think the Mercedes domination did not finish," Alonso said. "I think they are very, very strong.

"They are doing some interesting days. With that amount of laps it means you have very clear things so when they put the potential as well, they are stronger than ever probably - even more than the last two years."

Alonso managed to complete 119 laps on Tuesday - more than he managed in all of 2015 pre-season testing where McLaren endured endless problems.

"I am very proud of the team, of what we achieved in the last couple of months," he added.

"I enjoyed today but I think there is performance we need to unlock in the car."