BEIJING (Reuters) - Some Chinese buyers have canceled corn purchases from the United States and switched to rival supplier Ukraine, as Beijing tightens controls on processing genetically modified strains of the crop, three trade sources and an analyst told Reuters.

FILE PHOTO: A harvester unloads corn to a cargo truck at a farm in Gaocheng, Hebei province, China, September 30, 2015. REUTERS/Kim Kyung-Hoon/File Photo

Any prolonged shift by one of the world’s top corn importers would unnerve U.S. farmers as they prepare to harvest a bumper crop this year and could potentially mark a new front in trade tensions festering between China and the United States.

Chinese buyers late last year stepped up purchases of U.S. corn, which is mostly genetically modified, following a rally in domestic prices.

But the sources said it had become tougher for Chinese grain mills to get permits to process genetically modified corn this year, forcing some traders that supply them to instead turn to non-GMO shipments from Ukraine.

China’s agricultural ministry did not respond to a request for comment, while the trade sources declined to be identified as they were not authorized to speak with media.

“Traders this year have been more inclined to buy corn from Ukraine as it is non-GMO and doesn’t require the (processing) permit,” said Cherry Zhang, analyst with Shanghai JC Intelligence.

“Even some who had ordered U.S. corn canceled the orders and switched to Ukraine.”

It was unclear how many shipments had been affected, but one of the sources, a senior trader in Beijing, said up to four cargoes totaling about 210,000 tonnes and worth about $40 million based on current prices had been canceled last month as end-users had not received permits to process GMO crops.

“For March (delivery), some people had no choice, they had to wash out, but for April and May, they are still waiting (to see if their buyers get processing permits),” he said, using an industry term for cancelling an order.

The development comes as Beijing broadly increases scrutiny of genetically modified imports. Last year, it toughened the application process for getting safety certificates for imports of GMO soybeans, delaying cargoes and curbing purchases.

Ukraine accounted for about two-thirds of China’s 2.83 million tonnes of corn imports last year, with the United States making up a quarter.

Export prices from Ukraine have jumped on the higher demand, the senior trader said, rising by $20 since the start of the year to this week hit their highest since July 2016 at $180 per tonne.

“It is hard for U.S. corn to come in now due to the GMO issue. Getting the GMO processing permit is very difficult,” said a corn trader in eastern China who canceled a U.S. cargo.

“I had only ordered 7,000 tonnes of U.S. corn, but had to cancel the order and turned to Ukraine,” he said.

On Sunday, Beijing launched an anti-dumping and anti-subsidy investigation into imports of sorghum from the United States, fuelling concerns in the industry that soybeans might be caught up in trade action.