However, the State Department also singled out the country's epidemic of forced labor, in which both internal and external migrants are conscripted to work in coal mines or factories without pay, as well as its continued use of re-education hard labor camps for political dissidents.

In Russia, there are estimates that 50,000 children are involved in involuntary prostitution, said David Abramowitz, vice president for policy at Humanity United, an advocacy group. What's more, about one million people there are thought to be exposed to exploitive labor conditions, including extremely poor living conditions, the withholding for documents, and nonpayment for services.

"In 2012, the government deported hundreds of labor trafficking victims found in squalid conditions in a Moscow garment factory and levied criminal charges against other trafficking victims allegedly held in servitude for a decade," the report found.

Human Rights Watch has pointed out that some of Russia's labor abuses have occurred during the preparations for the 2014 Winter Olympics in Sochi, with some workers enduring "12-hour shifts with one day off per month, having their passports confiscated, being denied employment contracts, and facing unsanitary and overcrowded employer-provided accommodations, with up to 200 migrant workers living in a one single-family home."

Still, prosecutions for human trafficking there remain low compared to the scope of the problem, and the government has still not established any concrete system for identifying or helping trafficking victims, the State Department found.

In Uzbekistan, the annual cotton harvest has been the biggest human-trafficking culprit. The country is the world's sixth largest cotton producer, and each year local officials force thousands of children to pick cotton in the fields in order to meet quotas cheaply. (Abramowitz points out that this year, the country actually replaced some of the child workers with teenagers and city workers, but it's forced labor nonetheless.)

Abramowitz said the three countries could improve their "tier" ranking if they stepped up their training of law enforcement officials to identify and assist trafficking victims, cracked down on forced labor in factories, and became more transparent about their data on sex slavery and other human rights abuses.

Russia and China reacted angrily to the report, saying the rankings were based on diplomatic closeness with Washington rather than realities on the ground.

"The very idea of raising this issue causes indignation," Russian foreign ministry human rights envoy Konstantin Dolgov said in a statement.

Beijing's foreign ministry spokeswoman Hua Chunying said, "We believe that the U.S. side should take an objective and impartial view of China's efforts and stop making unilateral or arbitrary judgements of China."

In a forward to the report, Secretary of State John Kerry wrote that fighting human trafficking would remain a foreign-policy priority. "Fighting this crime wherever it exists is in our national interest," he wrote.