Mercedes's two drivers have admitted that the team are struggling with tyre problems in 2017.

After three races, Ferrari's Sebastian Vettel is leading the world championship, leaving triple back-to-back champions Mercedes to ponder the reasons.

"We have a bit to do at this test," Lewis Hamilton is quoted as saying by DPA after staying around in Bahrain for the post-race test that began on Tuesday.

"We need to improve the car and our knowledge about the tyres so that we are in a better position in the next race."

The next race in Russia should at least be cooler, with Mercedes more comfortable in China than in hotter conditions like Melbourne and Bahrain.

"The softer compound has been more of a struggle with the tyres and also the hotter it is, more of a struggle," Valtteri Bottas admitted. "So it's definitely something for us to understand."

The Russian Grand Prix takes place on April 30.