Thailand’s seafood industry defended itself Tuesday against allegations that it uses forced and child labor, after the U.S. State Department last week downgraded the country’s ranking to the lowest level in its annual human trafficking report.

The Southeast Asian nation exports more than $2 billion worth of shrimp each year, much of it to the U.S., where media reports in the Guardian and other publications have brought wider public attention to allegations of forced labor in Thailand’s fishery industry, particularly in the export-focused shrimp sector. The U.S. government’s move to rank Thailand alongside North Korea, Cuba and Iran in the lowest “Tier 3” ranking in its annual Trafficking in Persons Report has further focused attention on the issue and created a public relations headache for Thailand’s military leaders, who seized power in a coup d’état last month.

Leading figures in the seafood sector insist that there are no instances of child or forced labor in the industry. Panisuan Jamnarnwej, president emeritus of the Thai Frozen Foods Association, said he was baffled by media reports describing slavery-like conditions.

“We do not want to single out which reports have tried to attack us,” Mr. Panisuan said. “But right now I can confirm that there is not any use of illegal labor.” His comments came in the context of questions about labor conditions in land-based shrimp factories.

Poj Aramwattananont of the Thai Fishery Producers Coalition said he was concerned that the U.S. and other consumers might opt to boycott Thai shrimp if the industry can’t clear its name.

French supermarket firm Carrefour CA.FR +1.78% and Norway’s ICA have suspended Charoen Pokhphand Foods from their list of shrimp suppliers after the Guardian reported that “trash fish” caught using forced labor was used as feed on shrimp farms. CP Foods has since said it would audit its entire supply chain.

Other customers, notably Costco Wholesale Corp., have maintained orders, and Thai executives said they intend to visit the U.S. to assure customers that the industry doesn’t use illegal labor and complies with local and international labor laws. “Costco is scheduled to visit us at the end of this month to receive all documents to help them explain the situation to their clients,” Mr. Poj said.

The U.S. report criticized Thailand for taking little action to rectify the problem. It referred to a 2009 court case in which two people were found guilty of subjecting 73 victims to forced labor at a shrimp-peeling factory, criticizing authorities for treating the case as a civil rather than a criminal violation. The downgrade to Tier 3 status means that the U.S. can withhold some non-humanitarian and non-trade-related aid.

The leader of the Thai junta, Gen. Prayuth Chan-ocha, on Friday said the military would crack down on anyone trafficking in forced labor, and referred to what he described as influential individuals involved in the trade. The military government, he said, must deal with the problem swiftly to “bring back Thailand’s credibility on human rights and labor issues, and raise them to international standards.”

The slavery allegations come at a difficult time for Thailand’s shrimp industry. The country was for years among the world’s largest exporters, but a bacterial infection began ravaging shrimp stocks in 2012. Known as early mortality syndrome, or EMS, the disease kills shrimp before they can reproduce. The United Nations says the infection poses no threat to human health, but no effective treatment has been found.

In 2013, Thailand’s shrimp output fell 54% to 250,000 tons compared with the previous year, according to the Thai Shrimp Association. As a result, Thailand’s share of the U.S. shrimp market fell to 22% in 2013 from 36% in 2011.

“With the supply chain problem still unresolved and the probable impact from the U.S.’s Trafficking In Persons report, Thailand’s total shrimp exports are expected to contract more than 22% this year,” said Kevalin Wangpichayasuk, an assistant managing director at Bangkok-based Kasikorn Research Center.

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