The state-owned Citic Resources Holdings Ltd. 1205.HK +1.60% said about half of the alumina stockpiles it had stored at China's Qingdao port couldn't be located, heightening concerns over the use of commodities for financing in the country.

Citic Resources, a mining and trading company, said this month that it had applied to a court in Qingdao, a port on China's eastern coast, for help in securing metals it owns in warehouses. Citic Resources' parent is Citic Group, one of China's largest state-owned companies and a big financial concern.

Citic Resources, in a statement released Wednesday to the Hong Kong stock exchange, said the court couldn't locate 123,446 metric tons of alumina, a mineral used to produce aluminum. Citic Resources said it stored 223,270 tons of alumina and 7,486 tons of copper at the port that was awaiting delivery to buyers. The company said it would now conduct its own investigation into the missing commodities. At current market prices, the missing alumina is worth about $50 million.

Qingdao courts didn't respond to calls for comment.

The statement came as Western and Chinese lenders are looking into suspected fraud in China involving metals that were used as collateral. Banks have lent hundreds of millions of dollars to Chinese commodities traders in recent years, using commodities such as copper, iron ore and aluminum as collateral.

Western lenders say they are trying to determine whether metals stored at Qingdao port as collateral against loans were illegally pledged by a Chinese trading firm to more than one lender to obtain multiple loans. Banks that have made loans backed by collateral in Qingdao port include Citigroup Inc. C -0.77% and Standard Chartered STAN.LN -0.51%PLC, according to executives with Western banks. Both banks have acknowledged potential problems with commodity financing in China but have given no further details.

Qingdao Port International Ltd. 6198.HK +0.88% , which operates the port, on June 6 said Chinese authorities were conducting a probe into metals stored there. Chinese authorities haven't publicly commented on the probe. Port officials didn't respond to calls for comment Wednesday.

It isn't clear if Citic Resources' request for court assistance was linked to Decheng Mining Ltd., a Qingdao-based metals trader. Bankers are looking into whether entities linked to Decheng illegally pledged the same stocks of commodities multiple times as collateral to get loans from banks, according to executives at Western banks who made some of the loans.

Qingdao port's statement didn't name Decheng or any related companies. Attempts to reach officials at Decheng haven't been successful.

Meanwhile, foreign banks are pulling back on loans backed by commodities as collateral, according to Western bankers. Other banks are seeking to secure additional protection from borrowers and moving collateral to more secure locations, including ports outside China.

"Commodities financing for sure is going to have a rethink," said one executive at a Western bank involved in the probe. "There aren't too many banks out there that are open for business."

Western bankers complain that they still can't access the storage facilities at Qingdao to check on the collateral promised to them in return for loans. Banks say they also are concerned about collateral held at Penglai, another port some 150 miles north of Qingdao. Inspectors for the banks were denied access to a warehouse at Penglai and had to take photographs of aluminum stockpiles from outside the port's gates, according to a person familiar with the matter.

Copper prices on the London Metal Exchange shed 5% in the last two weeks as a result of fear companies may have to sell collateral to pay back loans, dumping metals onto the market. But prices have stabilized recently, rising 0.2% Tuesday to close at $6,704 a ton. Prices in Shanghai have risen 1.3% so far this week, after falling 2.3% in the preceding two weeks.

Some analysts said the market was betting the problems in the sector aren't widespread.

"Metal markets haven't been seriously affected by the news, as it seems for now that only some foreign banks are affected," said Hou Jing, an analyst with Shanghai CIFCO Futures Ltd., a commodity trader. Ms. Hou noted that Citic's disclosure didn't move markets by much on Wednesday.

Others say prices could fall again in the future if China's probe spreads to larger ports such as Shanghai.

"If news comes out that more copper has been pledged multiple times or the investigation against shadow banking is widened to other ports, markets will certainly be jittery," said Helen Lau, an analyst at UOB Kay Hian.