Chinese importers have defaulted on at least 500,000 tons of US and Brazilian soybean cargoes worth around $300 million, as they struggle to get credit due to losses in processing beans, trade sources said on Thursday.

Three companies in East China's Shandong Province had defaulted on payments for the shipments as they were unable to open letters of credit with banks, said one Beijing-based source.

A string of defaults on loans, bonds and shadow banking products in recent weeks has highlighted rising credit risks in China, partly fueled by signs the economy is slowing.

Semiconductor, software and commodities firms are among the most at risk of credit defaults, a Reuters analysis showed.

Up against a cooling Chinese economy and signs that authorities will not step in every time a loan goes bad, banks are becoming more hard-nosed and selective about whom they lend to.

"There are five to six (panamax) cargoes which are unable to be unloaded at ports because buyers cannot open LCs (letter of credit) and there are no LCs for an additional 5 to 6 cargoes floating on the sea," the Beijing-based source said. Each panamax cargo is for 50,000 to 60,000 tons.

Defaults by buyers in China, which imports 60 percent of the soybeans traded in the world, would likely cap a rally in global prices as it coincides with bumper supplies from Brazil and Argentina hitting the market.

With negative processing margins and tightening credit, sources said there could be more defaults on cargoes of soybeans, crushed to make cooking oil and animal feed ingredient soy meal.

Crushers are losing 500 yuan to 600 yuan ($81 to $97) for processing a ton of soybeans, compared with a 600 yuan profit in the fourth quarter of last year during peak consumption and when some shipments were delayed.

The fat margin in the fourth quarter prompted China to purchase 27.7 million tons of US soy so far in the current marketing year to August 2014. China bought a total of 21 million tons of US soybeans the year before.

China imported 15.35 million tons of the oilseed in the first quarter, up 33.5 percent on a year earlier, according to official data issued on Thursday.

"Crushers are making big losses while downstream product meal is not selling very well," said an official at a body, which oversees soybean imports under theMinistry of Commerce.

Imports could fall below 15 million tons in the third quarter from 18.25 million in the same period last year, traders and industry officials said.