Russia spoke of its "indignation". A Chinese official called the report an example of an "arbitrary judgement".
The annual US Department of State report relegated Russia and China into its lowest category, which also includes Iran and North Korea.
Uzbekistan also fell to Tier 3 in the report, published on Wednesday.
The 21 countries in Tier 3 may face sanctions in areas including cultural and education programmes, and the US could withdraw its support for loans from the World Bank or International Monetary Fund.
"The very idea of raising this issue causes indignation," Russian foreign ministry human rights envoy Konstantin Dolgov said in a statement.
It said the report had used "unacceptable methodology", grouping countries in terms of their friendliness with Washington.
"In fighting organised crime, including countering trafficking, Russian authorities will never follow instructions worked out in another country, let alone fulfil conditions presented nearly in the form of an ultimatum," it added.
In Beijing, foreign ministry spokeswoman Hua Chunying said China attached "great importance to fighting all crimes of trafficking".
"We believe that the US side should take an objective and impartial view of China's efforts and stop making unilateral or arbitrary judgements of China," she said.
The US report said Russia had made "ad hoc" efforts to tackle trafficking, but that it "had not established any concrete system for the identification or care of trafficking victims, lacking any formal victim identification and referral mechanism".
It said China had taken steps to raise awareness and work with international organisations, but drew attention to continuing cases of child and adult forced labour, and sex trafficking of women and girls.
"Despite these modest signs of interest in anti-trafficking reforms, the Chinese government did not demonstrate significant efforts to comprehensively prohibit and punish all forms of trafficking and to prosecute traffickers," the report said.
It said trafficking was "pronounced" among China's internal migrant population, and that the country's one-child policy had produced more boys than girls, fuelling demand for prostitution and foreign brides.
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