Media playback is unsupported on your device Media caption Zhang Qingqing's family told her she was bought as a baby

China "is not making significant efforts" to stop human trafficking, the US says, claiming that fewer people are now being prosecuted than before.

The US Department of State released its annual Trafficking in Persons Report on Thursday, and downgraded China to one of the worst offenders.

The reports highlights the treatment of North Koreans who may have been trafficked and then sent home by China.

There has been no response from China, which could now face sanctions.

Countries placed in the third tier of three in the report - including North Korea, Sudan and Venezuela - can lose non-humanitarian aid. But Associated Press reports that presidential waivers mean Tier Three countries do not always get punished.

Afghanistan, Qatar and Malaysia were upgraded to Tier Two as they were seen to be making efforts to crack down on the practice, and improve conditions for those who have been trafficked.

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US Secretary of State Rex Tillerson said China was downgraded "in part because it has not taken serious steps to end its own complicity in trafficking, including forced labourers from North Korea that are located in China".

Too often, the report says, China sends North Koreans home without having screened them for signs of human trafficking - even though they could face torture or execution on their return.

Mr Tillerson said an estimated 50-80,000 North Koreans were working overseas in forced labour, often up to 20 hours a day.

Media playback is unsupported on your device Media caption Mobile app to tackle China child trafficking (2017 video)

While the report comes from the Department of State rather than the White House, it is nevertheless the most significant rebuke against the Chinese government by the US since President Donald Trump took office in January.

However, Reuters reported that Mr Trump was becoming "increasingly frustrated" over Chinese inaction on North Korea, and that he was considering trade actions in response.

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Among the other findings on China highlighted in the report:

reports continue to emerge from the restive Xinjiang province that say Uighur Muslims are being put into forced labour by officials "despite the local government issuing a notice in early 2017 that the practice had been completely abolished";

fewer sex and labour traffickers were prosecuted in the past year than the year before;

the government did increase co-operation with foreign governments over trafficking, and to investigate cases of trafficked Chinese nationals abroad.

The report, which covers 180 countries, is billed as the most comprehensive resource of efforts being done to stop trafficking.

The North Korea issue - Barbara Plett Usher, BBC State Department correspondent

China was among 21 countries downgraded by the State Department, perhaps for good reason.

Beijing made it off the lowest rank of the US human trafficking index four years ago, but has hovered just above it ever since.

This year it was determined that Beijing simply wasn't doing enough to protect victims and prosecute traffickers. But it's also true that the political preoccupation of the moment is North Korea.

The Trump administration has become concerned about North Korean labourers who are forced to work abroad, many in China, with their pay directly funding the leadership in Pyongyang. So it was interesting that Secretary Tillerson chose to highlight this issue in his comments about China's record.