BEIJING (AFP) - China and Russia on Thursday attacked a US government report that accused them of failing to do enough to combat human trafficking, a finding that could trigger sanctions by Washington.

"We believe that the US side should take an objective and impartial view of China's efforts (in fighting human trafficking) and stop making unilateral or arbitrary judgements of China," foreign ministry spokeswoman Hua Chunying said at a regular briefing in Beijing.

"The Chinese government always attaches great importance to fighting all crimes of trafficking," she said.

Moscow was also furious at the US report, which it decried as politically motivated.

"As far as the application of unilateral sanctions against Russia is concerned... the very idea of raising this issue causes indignation," the foreign ministry's human rights envoy Konstantin Dolgov said in a statement.

The United States is often at odds with China and Russia - geopolitical heavyweights which along with Washington are permanent members of the United Nations Security Council - on human-rights issues.

They cooperate in some areas, such as pressuring North Korea to give up nuclear weapons. But in others, such as the Syrian civil war, Beijing and Moscow have been at defiant odds with Washington.

The US State Department on Wednesday downgraded the two countries, as well as Uzbekistan, to the bottom of a table on human trafficking.

The three nations had languished for years on the "Tier 2 Watch List", having been granted past waivers amid promises to do better.

But under the terms of a 2008 law, they could no longer stay on the watch list and either had to move up a level or be downgraded. The new report took the latter step, which could now trigger cuts in non-humanitarian and non-trade US aid.

President Barack Obama will determine whether to enact any sanctions against the three nations in September.

The report found that "trafficking is pronounced among China's internal migrant population" and "forced labour remains a problem, including in brick kilns, coal mines and factories".

China's one-child policy has resulted in "a skewed sex ratio of 118 boys to 100 girls in China, which served as a key source of demand for the trafficking of foreign women as brides for Chinese men and for forced prostitution".

Beijing had failed to "demonstrate significant efforts to comprehensively prohibit and punish all forms of trafficking", the report maintained.

Congressman Chris Smith, who has authored key legislation on trafficking, said China has become the "sex and labour trafficking capital of the world".

"Women and young girls have been - and are today being - reduced to commodities and coerced into prostitution," the Republican said in a statement.

Ms Hua, the foreign ministry spokeswoman, defended China.

"We have been constantly improving our domestic legislation, strengthening our law enforcement and judicial measures, and carried out cooperation with all countries, in particular our neighbouring countries," she said.

"China has made solid efforts and achieved remarkable progress in fighting domestic and transnational trafficking."

The State Department report said that in Russia, a million people "are exposed to 'exploitative' labour conditions characteristic of trafficking cases, such as withholding of documents, non-payment for services, physical abuse, or extremely poor living conditions".

Mr Dolgov said the report was unbalanced and poorly conceived.

The report should have taken a "deep and objective review of why human trafficking is growing, including on the territory of the United States", the Russian envoy said.

But instead, "the report's authors are once again using unacceptable methodology under which governments are ranked based on the political sympathies or antipathies of the US State Department".