The United States on Thursday downgraded its assessment of Japan's efforts to meet minimum standards for the elimination of human trafficking, citing concerns over the abuse of labor migrants working in the country.

In its annual Trafficking in Persons Report, the State Department placed Japan in Tier 2, after the country earned the highest classification for two consecutive years through 2019 in the four-tier list.

Countries with poor human rights records including China, Iran, Myanmar, North Korea, Russia, Syria and Venezuela remained on Tier 3, the lowest level of the rating system which also includes a Tier 2 Watch List.

On Japan, the report said the government is making "significant efforts" to meet the minimum standards to tackle trafficking, such as by identifying more victims than the previous year and increasing on-site inspections of businesses employing migrant workers.

"However, these efforts were not serious and sustained compared to those during the previous reporting period," the report said, citing that authorities failed to identify "a single trafficking case" in connection with foreigners working under the country's technical intern program "despite persistent reports of forced labor."

Japan has introduced the program in the name of transferring skills to developing countries. Interns sent to Japan have helped to fill serious labor shortages in the graying country that has taken a cautious stance on immigration.

But critics say there are suspected abuses of such workers, including unpaid wages and illegal overwork.

Despite the "prevalence of forced labor indicators" identified through inspections by an oversight mechanism, the Japanese government "did not report prosecuting or convicting any individuals for involvement in the forced labor" of technical interns, the report said.

It also pointed to lingering concerns over Japan's new visa system introduced in 2018. Under the system, technical interns have been allowed to switch their visas to the newly created ones, enabling them to extend their stays.

"Although there were no reported cases of forced labor within this system in 2019, observers continued to express concern that it would engender the same vulnerabilities to labor abuses, including forced labor," as those inherent to the technical intern program, the report said.