JAKARTA - The Indonesian navy has impounded seven ships that were reportedly attempting to smuggle thousands of tonnes of sea sand from Riau province. Colonel Adyaman A.S. told the Antara national news agency that the ships did not have proper documentation to export the sand that was taken from the province. 'We are still studying the exact amount of sea sand that the ships had been trying to smuggle abroad,' he said yesterday. Indonesian officials estimate that every year sand worth US$200 million (S$350 million) is smuggled to buyers in countries like Singapore and elsewhere. They say upwards of 300 million cubic metres of sand is removed from Riau every year. The loss of the sand is fast becoming an environmental headache for Indonesia. Several of the vessels impounded by the Indonesian navy were foreign-flagged. Antara reported that one of them was flying the Russian flag, another had the South Korean flag and a third was flying the Japanese flag.