The second practice session for this weekend's United States Grand Prix has been cancelled after rain and thunderstorms hit the Circuit of the Americas in Austin.

Race control announced that the session was indefinitely delayed shortly before the action had been due to get underway, before abandoning the practice an hour later.

Fans in the grandstands were warned to take shelter from lightning, while the medical helicopter had to be grounded because of the conditions, which are forecast to continue throughout the weekend.

Mercedes's Nico Rosberg was fastest in the rain-affected first practice earlier.