US soybean exporters have received some warning signals on product quality from the world's largest importer, a month after US President Donald Trump's visit to Beijing secured them a juicy deal selling an extra 12 million tons of their produce to China.

On Dec 15, China's quarantine authorities said they had destroyed 6.8 tons of genetically engineered soybeans from the United States, citing mildew contamination as the reason, according to the bulletin on November substandard agricultural imports released by China's General Administration of Quality Supervision, Inspection and Quarantine.

Another complaint is impurities in US soybeans, which are more often higher in shipments from the eastern US than those from the West Coast, according to a quarantine official at the Chinese Embassy in Washington.

To address the problem, the US Department of Agriculture has said that shipments with impurity levels below a new standard of 1 percent, half the current level, will receive priority for shipment, while soybeans above 1 percent may be held back for more cleaning, Bloomberg News reported on Wednesday.

"The new standard may go into effect on Jan 1," the report citied USDA spokesman William Wepsala as saying. The report said China's General Administration of Quality Supervision, Inspection and Quarantine made the requirement in early December, a move some US analysts said may "curb some American shipments".

"We welcome the US Agriculture Department's move to improve quality for products bound for China, and it is normal to ensure exports meet the needs of the importing nation," said Wei Zhenglin, an agricultural attach?? with the Chinese embassy on Thursday.

Any measures taken to ensure the quality standards of soybeans will contribute to consumers' food safety and at the same time help make the trade sustainable, Wei said.

China's soybean imports totaled 77.3 million tons in the first 10 months of the year, up by 15.2 percent from the same period last year, according to customs statistics. Of the total, 22 million tons, or 28 percent, were from the US.

That figure is expected to surge.

During Trump's state visit to China in November, Chinese buyers committed to buying an extra 12 million tons or $5 billion worth of soybeans from the US in the 2017-18 seasonal year. It was part of the $253.5 billion worth of economic and trade agreements reached.

In its latest outlook, the USDA predicted US soybean export volume will continue to set records, raising the soybean forecast $200 million to $24.1 billion for fiscal 2018. It cited competition from South America and demand in China as the key drivers in its forecast.

Last year, China imported 83.23 million tons of soybeans from the rest of the world, including 33.66 million tons, valued at $13.8 billion, from the US, accounting for 40.7 percent of China's total imports, according to Chinese customs statistics.

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