Max Verstappen fumed at "stupid idiot" Lewis Hamilton and aimed a middle-finger salute at his title rival in a heated United States Grand Prix session.

The fight for the Formula One title intensified when the pair came close to making contact during FP2 at the Circuit of The Americas on Friday.

Championship leader Verstappen snapped over the team radio after going wheel-to-wheel with Hamilton, saying: "Ha! Stupid idiot!" and making his feelings clear with a gesture to the Mercedes driver.

A Red Bull engineer told Verstappen to "ignore it" as the Dutchman endured a frustrating day, having been caught in traffic at the start of the mid-session short runs.

He could only finish down in eighth in the second session as Sergio Perez set the pace, with Hamilton third behind Lando Norris.

Valtteri Bottas topped the timesheets in the first session, but will take a five-place grid penalty in Austin after having a sixth engine of the season – with three being the limit – fitted and a sixth exhaust.

Silver Arrows team principal Toto Wolff says there is a risk that Hamilton, who trails Verstappen by only six points, could take another grid penalty this season

He added: "I can't say whether we will be taking one and what the percentage is, but obviously the risk is still there.

"What is difficult to evaluate is do you want to pre-empt the situation and take another penalty and take the hit or do you want to really run it and possibly risk a DNF, and that is a discussion that is happening as we speak, and we haven’t come to the right answers yet."

Williams driver George Russell, Bottas' replacement at Mercedes for 2022, and Aston Martin's Sebastian Vettel will start on the back row after they had multiple new engine parts fitted.

Valtteri Bottas, George Russell and Sebastian Vettel have taken grid penalties for the United States Grand Prix this weekend.

Bottas won in Turkey last time out, but a repeat would appear unlikely at the Circuit of The Americas on Sunday after the Mercedes driver had a sixth engine of the season – with three being the limit – fitted and a sixth exhaust.

The Finn, who sits third in the driver standings and was fastest in the first practice session on Friday, will take a five-place penalty for the race in Austin.

Silver Arrows team principal Toto Wolff said: "I think you see that we are suffering with reliability this year, we're going onto the sixth engine for Valtteri, and it is not something we choose to do, but on the contrary; we are trying to really get on top of the problems, and we haven't understood fully [what they are]."

Wolff says there is a risk that Lewis Hamilton could take another grid penalty this season as he battles for the title with Max Verstappen.

He added: "I can't say whether we will be taking one and what the percentage is, but obviously the risk is still there.

"What is difficult to evaluate is do you want to pre-empt the situation and take another penalty and take the hit or do you want to really run it and possibly risk a DNF, and that is a discussion that is happening as we speak, and we haven’t come to the right answers yet."

Williams driver Russell, Bottas' replacement at Mercedes for 2022, and Aston Martin's Vettel will start on the back row after they had multiple new engine parts fitted.

Hamilton was 0.045 seconds slower than his team-mate Bottas in FP1, with his championship rival Verstappen third-quickest – almost a second slower than Bottas.