The US embassy in Saudi Arabia has joined Britain and Australia by warning that terrorist groups in the kingdom are still active.
It is feared they may be planning attacks to coincide with the holy Muslim month of Ramadan, which starts this week.
Earlier, Britain's Foreign Office warned that terrorists could be in the final stages of planning attacks in Saudi Arabia. It advised UK citizens not to travel there.
Australia issued a similar warning on Thursday. There is speculation that the plot is linked to al-Qaeda.
Travellers have been told to be vigilant, especially in public places popular with foreigners such as hotels and shopping malls.