The computer worm was first seen in Ukraine on Tuesday, going on to affect port facilities ranging from Mumbai to Los Angeles. It hit Argentina on Wednesday, slowing wheat and fertiliser shipments and threatening to impact the flow of soybeans to the country's main client, China, at the height of export season.

"Cofco's system has been affected by the global attack. It has been infected by a virus. So they are working mechanically, not connected to their regular information system," Guillermo Wade, head of Argentina's CAPyM port operators' chamber, told Reuters in a telephone interview.

A local Cofco representative did not respond to a request for comment. The company's Brazil unit declined to comment.

In 2014 Cofco agreed to buy Dutch grain trader Nidera and the agribusiness of Noble Group for more than $3 billion. The deal propelled Cofco to the No. 2 spot among exporters of Argentine grains, oilseeds and byproducts.

"So far we've heard only Nidera and Noble were hit," said a senior Buenos Aires-based grains trader, who requested anonymity.

One of several ships scheduled to unload fertiliser at a Cofco port facility in Rosario had been halted since Tuesday, Wade said, while a vessel loading wheat produced in the vast Pampas grains belt was interrupted for hours until local operators found a mechanical work-around to the attack.

"It is affecting all of Cofco's port operations in Rosario," Wade added. "The cargo ship that was unloading fertiliser is still stopped. This has caused a backup in the line of other ships that have cargo to unload."

Cofco is one of 43 export and port service companies that belong to the CAPyM chamber.

Cofco operates two ports in Rosario. Each has two berths used to load grains, oilseeds and byproducts, as well as unload fertilizers used by growers across the country.

Argentina is the world's top exporter of soymeal livestock feed and the No. 3 supplier of raw soybeans, as well as a major global corn supplier.

Other major exporters like Archer Daniels Midland Co (>> Archer Daniels Midland Company) have operations in Rosario. The port complex is located on the banks of the Parana River, which leads out to the shipping lanes of the South Atlantic.

On Tuesday U.S.-based ADM spokeswoman Jackie Anderson said a small number of the company's computers were impacted by the cyber attack. "We have been able to continue normal business operations and are continuing to monitor the situation," she said.

(Additional reporting by Jonathan Saul in London, Marcelo Teixeira in Sao Paulo and Karl Plume in Chicago; Editing by Bill Rigby)

By Hugh Bronstein

Stocks treated in this article : Archer Daniels Midland Company, AP Moller-Maersk