China has imposed tougher restrictions for imports of live pigs from the U.S. amid concerns about a fatal swine virus, according to the U.S. Department of Agriculture and livestock-industry officials.

China, the world's largest consumer of pork, has asked the USDA to conduct testing and provide certification that live hogs come from herds that are free of porcine epidemic diarrhea, or PED, virus. The disease has expanded to nearly 30 U.S. states and killed millions of young pigs since it was first identified in the country last spring, according to industry estimates.

Live hogs represent a relatively small portion of pork-related U.S. exports to China. The Asian nation last year bought 14,000 hogs, valued at $20 million, according to the USDA. In contrast, exporters shipped $703.5 million worth of fresh pork to the country, according to the U.S. Meat Export Federation, a trade group.

A spokesman for the USDA's Animal and Plant Health Inspection Service said the agency "is both capable and willing" to comply with China's new requirement for testing and certification. But, he said, the government also would "continue to work with the Chinese" to try to eliminate the requirement.

Some U.S. livestock exporters said Friday that China had denied permits for live-hog imports from the U.S. until the USDA implemented the testing and certification procedures.

A representative of the Chinese Embassy in Washington couldn't immediately be reached for comment on the country's policy.

Scientists say the virus, which is closely related to a strain found in China in 2012, is fatal only to young pigs and poses no threat to human health or food safety.

Typically, live hogs shipped from the U.S. to China are breeding animals that help supply pigs for the country's commercial pork industry, said Jay Truitt, a representative for the Livestock Exporters Association of the USA, a trade group in Washington, D.C.

Tony Clayton, president of Clayton Agri-Marketing Inc., a Missouri-based livestock exporter, said he learned Wednesday that China had stopped issuing permits for live-hog imports, and he has already begun testing pigs for the virus. "PED is an animal health issue that we're all trying to work around," he said.

Mr. Clayton said his company is already required by China and other importers to test for common swine diseases. Pigs are required to remain in quarantine for 45 days of additional testing once they enter China, he said.