US citizens may not open an account. In addition, US residents* (permanent residents or those whom have stayed in the US for more than 183 days over the last 3 years) may not open an account.
*US residents generally mean those who have stayed in the United States for at least 31 days of the current year and have spent 183 or more days in the US over the past three years. The method for calculating the length of stay in the US for the last three years is the sum of number of days in the current year, 1/3 of the number of days from last year, and 1/6 of the number of days from the previous year. In addition, permanent residents are considered US residents.