AUSTIN, TEXAS (REUTERS) - Lewis Hamilton's Mercedes team mate Valtteri Bottas will have a five-place grid penalty for Sunday's US Grand Prix after taking his sixth engine of the Formula One season.

Drivers are allowed only three engines without penalty and Bottas has already had grid drops at the Italian and Russian Grands Prix.

The fourth new engine carried a 10-place penalty but subsequent additions incur five.

A Mercedes spokesman said the decision to change the engine at Austin's Circuit of the Americas was to provide the best balance of performance and reliability for the final six rounds of the season.

Seven times world champion Hamilton, who is six points adrift of Red Bull's Max Verstappen, took a 10-place grid drop at the previous race in Turkey after exceeding his engine allocation.

Bottas won that race from pole position.

Other Mercedes-powered teams have also had problems this season with a power unit whose reliability has come into focus after years of domination.

Aston Martin's quadruple champion Sebastian Vettel is set to start from the back of the grid at the Texas track after a new Mercedes engine, his fourth of the season, was put in his car along with other power unit components.

So too will Williams' George Russell for the same reasons.