Russia has raised 'further issues' with a trade agreement that was supposed to have restarted US poultry exports to the country after a six-month ban, according to US chicken industry sources, reports Moscow Times.

Industry and government sources did not specify the issues raised. The matter has reached Washington, where US Agriculture Secretary Tom Vilsack said he was "concerned" and monitoring the situation.

A US Department of Agriculture spokesperson later said the department was working with the United States Trade Representative "to ensure the terms of the agreement are fulfilled".

Russia had been the largest US export market for chicken until it banned the meat in January, claiming a chlorine disinfectant used on the meat violated its food safety policy.

A deal to restart shipments was reached in June between US President Barack Obama and President Dmitry Medvedev. As part of that deal, US chicken companies agreed to use non-chlorine treatments on chicken bound for Russia.

"The Russians are now raising further issues about the provisions of the agreement. We hope these matters can be resolved speedily and are awaiting further clarification from the US government," the National Chicken Council said in a statement on 4 August.

"It is not going as smoothly as we would like," Toby Moore, spokesman for the USA Poultry and Egg Export Council, said when asked about the latest issues with Russia. "It is in both governments' hands."

The deal came after a series of talks between US technical officials and the Federal Consumer Protection Service.

The USDA began posting plants approved to ship to Russia in late July, according to Moscow Times, including some owned by top producers Tyson Foods, Pilgrim's Pride and Sanderson Farms.

Pilgrim's Pride sold out its entire August volume from Russian-approved plants, its chief executive said last week.

But there are now more technical issues, said Nikolai Vlasov, deputy chief of the Federal Veterinary and Phytosanitary Inspection Service, Interfax reports.

The service warned Russian importers to wait until it posted a list of US exporters on its web site.

Mr Vlasov said: "Resolving the problems has been complicated by a number of technical issues, and they've proven not so simple."