Seven Russian terrorism suspects once held at the US detention center at Guantanamo Bay, Cuba, suffered torture and other abuse after being returned home in 2004, Human Rights Watch alleges in a new report.

The seven, who were arrested in and around Afghanistan shortly after the 2002 US-led invasion and accused of fighting with the hardline Islamic Taliban regime, were repatriated with the guarantee that they would not be harmed.

"The Russian prisoners' experience illustrates why the United States should stop relying on 'diplomatic assurances' of fair treatment to justify sending prisoners from Guantanamo Bay to countries where they are at risk of torture," the rights watchdog said in a statement.

The seven repeatedly asked authorities at Guantanamo not to return them to Russia, saying they feared for their safety if sent back.

The 43-page report, which is based on interviews with three of the detainees, their families and lawyers, alleges that three of the seven suffered serious torture and ill-treatment after being arrested in Russia.

Two of them, the watchdog alleged, were convicted at unfair trials.

"The Russian experience shows why 'diplomatic assurances' simply don't work," said the report's author Carroll Bogert.

"Governments with records of torture don't suddenly change their behavior because the US government claims to have extracted some kind of assurance from them," she added.

Human Rights Watch said the United States was violating international law and its obligations under The Convention Against Torture by sending people back to countries where they were at risk of torture.

The New York-based group said a US government statement it received made it clear that Washington was aware of the threat of torture in Russia.

The watchdog said that other countries including Britain, Canada, Austria, the Netherlands and Switzerland were also attempting to deport or extradite terrorism suspects under similar diplomatic assurances.

"What happened to the former detainees is pretty standard for a lot of suspects in police custody in Russia," said Bogert.

"But that's just the point. The US government knew that these men would likely be tortured, and sent them back to Russia anyway," she said.

The watchdog urged the US government to establish screening procedures to allow those being transferred from Guantanamo Bay to challenge their transfers before an impartial body.