US customs officials at Miami international airport say they have seized several ancient stone carvings illegally exported to America from Cameroon using fake documents.

Cultural experts say the phallus-shaped sculptures - known as Ikom monoliths - were made between 200 and 1,000 AD near the town of Ikom in the Cross River State area of Nigeria, bordering with Cameroon.

They are each uniquely decorated with geometric patterns of human facial features and undeciphered inscriptions, which linguists believe is a prehistoric form of writing and visual communication.

US border officials say the stones will be returned to Cameroon.