Ferrari driver Fernando Alonso has clocked the fastest time in a disrupted first practice session ahead of the United States Grand Prix.

The start time was delayed initially by half an hour due to fog that had descended around the track in Texas, but it was not until 40 minutes later when drivers appeared.

Heikki Kovalainen was one of the first drivers to take to the circuit in the Lotus, while McLaren's Jenson Button was the first to set a lap time, moving him to the top of the standings, with his teammate Sergio Perez below.

Not long into the session, practice was halted due to a fault to the medical helicopter, which was nowhere to be seen for some time.

However, the aircraft did return, which allowed the session to restart, with Alonso eventually leading the timesheet and Button sitting just clear of the Spaniard.

First practice full standings:

1. Fernando Alonso (Ferrari)
2. Jenson Button (McLaren)
3. Valterri Bottas (Williams)
4. Esteban Gutierrez (Sauber)
5. Nico Rosberg (Mercedes)
6. Lewis Hamilton (Mercedes)
7. Felipe Massa (Ferrari)
8. Mark Webber (Red Bull)
9. Nico Hulkenberg (Sauber)
10. Pastor Maldonado (Williams)
11. Romain Grsojean (Lotus)
12. Sergio Perez (McLaren)
13. Heikki Kovalainen (Lotus)
14. Adrian Sutil (Force India)
15. Paul di Resta (Force India)
16. Daniil Kvyat (Toro Rosso)
17. Jean-Eric Vergne (Toro Rosso)
18. Sebastian Vettel (Red Bull)
19. Alexander Rossi (Caterham)
20. Max Chilton (Marussia)
21. Charles Pic (Caterham)
22. Rodolfo Gonzalez (Marussia)