Customs authorities have stepped up efforts to block illegal grain exports by traders hoping to cash in on high international prices.

As international rice and wheat prices have surged to historical highs, cases of grain smuggling have become more common, customs officials said.

Customs officials in Hangzhou, Zhejiang province, stopped four such attempts in the past several weeks, preventing nearly 7 tons of rice and 33 tons of wheat from being shipped out of the country illegally.

Officials in Ningbo, also in the eastern province, blocked more than 130 tons of wheat from being shipped out in four recent cases.

In March, international rice prices rose to their highest level in 19 years, and wheat prices rocketed to a record 28-year peak.

Meanwhile, domestic grain prices only experienced mild hikes, up 5.7 percent in January and 6 percent in February.

In the first two months of this year, the country exported 350,000 tons of rice, while it imported 138,588 tons, according to the latest statistics from the State General Administration of Customs.

The temptation of high international food prices spurred rice exports, as the high prices ate into imports, experts said.

The authorities launched a series of export restrictions late last year.

In December 2007, the authorities abolished export tax rebates for wheat, rice, corn and soybeans and at the beginning of this year, they began to levy a 5.25 percent provisional export tariff on 57 food stuffs, including wheat, corn, rice and soybeans.

In late January, the Ministry of Commerce announced that grain exporters would be given export quotas before signing new foreign export contracts this year.

Professor Wu Laping, from the China Agricultural University, said the gap between domestic and foreign food prices is growing.

"To guarantee a healthy domestic food supply, the government should certainly take measures to maintain stable prices and try to prevent the high food prices in the international market from having a huge impact on the domestic market," Wu said.

To promote agricultural production and ensure food security and market stability, the Ministry of Commerce recently issued a notice requiring local departments to strengthen supervision of the circulation of agricultural products and materials.

The ministry urged local government organizations to continue strengthening the reserves of important agricultural products, including meat and edible oil.

Large domestic supermarkets were asked to directly procure fresh agricultural products from agricultural enterprises and producers.