BANGKOK (Reuters) - Rice prices in the world's two leading producers of the grain, Thailand and Vietnam, are expected to surge after India's ban on exports of non-basmati rice, traders and officials said on Wednesday.

Traders said the ban might trigger a scramble among rice importers struggling to secure supplies and control soaring food prices. Supply concerns are intensifying as global stockpiles have halved and prices have doubled since last year.

"We have never seen before prices surging as high as the current market and I expect that prices could rise much higher," said Chookiat Ophaswongse, president of the Thai Rice Exporters Association.

The price of Thai 100 percent B grade white rice, which has risen more than 40 percent since early this year, is expected to rise by another 30 percent to $850 per tonne in April and May due to scarce supply.

Thai premium grade fragrant rice would also rise to an historic high $1,200 a tonne, double the $600 early this year, said Chookiat.

In October, India, which normally exports 4 million tonnes annually, imposed restrictions on certain grades of non-basmati rice.

That was relaxed after traders protested and the government allowed exporters to sell at a minimum $425 a tonne, but the measure still had a huge impact on global prices.

On Monday, India imposed a ban on non-basmati rice exports to ensure the country had enough rice to feed its more than 1 billion people and to ease pressure on domestic prices, which have pushed wholesale inflation to a 14-month high.

Sanjay Sethia, president of the All India Rice Exporters' Association, told Reuters on Tuesday that Indian rice exports could fall sharply to 250,000 tonnes this financial year, from as high as 5.5 million tonnes in 2007/08.

Vietnam, the world's second-biggest rice exporter, also has halted new exports deals for March and April shipment partly to implement contracts for the Phillippines, a major buyer.

But the country has no plans to raise rice export prices while the restriction on new contracts remained in place, an industry official and traders said on Wednesday.

Indicative prices have risen about 15 percent since early March after the Philippines bought rice in a tender last month at an average price of $708.04 per tonne, 49 percent above the price it had paid in January.

"Vietnamese rice prices could rise in line with global market moves after the limit is lifted, but we are still not clear by how much," a Vietnam Food Association official said.

Given the current curb on exports, Vietnam's 5 percent broken rice was offered only for reference at $620 to $650 a tonne, free-on-board Saigon Port, or 15 percent higher than a quotation of $550 a tonne in early March.

International rice prices have increased about 20 percent since the start of the year, according to the United Nations Food and Agriculture Organisation.

STRUGGLING EXPORTS

Given the surging prices, some Thai rice exporters have had to default on contracts to curb losses when they had to buy at higher prices for loading, while others have stopped selling rice altogether, waiting for the market to stabilise, Chookiat said.

"Exporters have losses of up to 10 billion baht ($317.5 million) right now and we agreed that we should slow exports in a bid to avoid losses," he said.

"I think I will wait until the dust settles before resuming selling rice, maybe when Vietnam relents on its export restriction," said a leading Thai exporter who declined to be named.

The Thai Rice Exporters Association has asked the government to set a ceiling for export prices to slow sales and help soften domestic prices, but the government has not yet responded.

Soaring prices have trimmed Thai rice exports in March and were expect to cut Thai shipments this year, Chookiat said.

"I think we could hardly achieve the target of 8.75 million tonnes this year," Chookiat said.

Thailand shipped 495,000 tonnes of rice in March, down from 985,000 tonnes in February, Commerce Ministry data showed.

In Vietnam, exporters are required to get the Food Association's clearance before loading.

"The government has said 'no' to signing of new contracts so we need to follow that for now," the Food Association official said.

Last week, Prime Minister Nguyen Tan Dung said Vietnam would cut rice exports by 22 percent this year as it fought to rein in soaring inflation at home and secure food supplies.