Washington said Wednesday that it had secured the return of the last prisoners in China classified by the State Department as wrongfully detained.

A source close to the matter said the arrangement was part of a swap deal with Beijing for three Chinese nationals in US custody who were not identified.

Beijing's foreign ministry confirmed on Thursday the return of three nationals from the United States, without specifying further details.

"Following unremitting efforts by the Chinese government, three Chinese nationals wrongfully imprisoned by the American side have now safely returned to the motherland," spokeswoman Mao Ning said at a regular press conference in Beijing.

"This shows once again that China will never abandon its compatriots at any time, and that the motherland is always there as a strong backup force for them," Mao said.

She added that Beijing "consistently opposes the American side carrying out the suppression and persecution of Chinese nationals out of political aims, and will continue as ever to take necessary measures" to uphold their interests.

A longtime "fugitive" was also extradited back to China, Mao said, adding that their return showed that "nobody can escape from the long arm of the law".