Rice loading at Indian ports has ceased, and there are now nearly one million tonnes of grain trapped there as buyers' refused to pay the government's new 20% export levy on top of the agreed contract price, five exporters told Reuters on Friday.  India, the world's largest exporter of the grain, tried to boost domestic supplies and stabilise prices on Thursday after planting was hampered by below-average monsoon rainfall by banning the export of broken rice and imposing a 20% duty on various other types.  "The duty became effective from midnight, but buyers are not ready to pay the duty," said B.V. Krishna Rao, president of the All India Rice Exporters Association (AIREA). "We have stopped loading vessels."  With large amounts loaded from eastern ports such as Kakinada and Visakhapatnam in Andhra Pradesh state, India exports two million tons of rice every month. In similar circumstances, New Delhi has in the past provided exemptions for contracts backed by letters of credit (LCs), or payment guarantees, issued until the day the government made a policy change, said Himanshu Agarwal, executive director at Satyam Balajee, India's biggest rice exporter. But that has not happened this time.  "Margins are wafer-thin in the rice business and exporters can't afford to pay 20% duty. The government should allow exports against already issued LCs," Agarwal said. When it forbade the export of wheat earlier this year, New Delhi permitted exports against already-issued LCs. According to traders, there are 750,000 tonnes of white rice at the ports that will be subject to a 20% duty starting on Friday. India has allowed loading the consignments that have been handed over to the customs, as for the broken rice ban. But loading needs to be completed before September 15. At least 350,000 tonnes of broken rice lying at various ports do not meet these criteria, and moving cargoes back to the hinterland is not possible, said a New-Delhi based dealer with a global trading firm. For transitional shipments of white rice and broken rice totaling 750,000 tonnes and 500,000 tonnes, respectively, the AIREA has requested that the government relax the new regulations. Given that India exports rice to more than 150 nations, any decrease in shipments would put more upward pressure on food prices, which are already on the rise as a result of the drought, heat waves, and Russia's invasion of Ukraine.