The US is "deeply concerned" about reports that detained Egyptian political activists have been abused and beaten by Egypt's security forces, US State Department spokeswoman Jen Psaki said on Tuesday.

Activists Ahmed Maher, Ahmed Douma and Mohamed Adel claimed during their trial on Monday that they had been beaten by security forces while being transferred from Tora Prison in southern Cairo to the courthouse a few kilometres away.

"If true, there is no justification for such treatment," Psaki declared during the State Department's regular Tuesday press briefing.

The defendants, who are charged with defying a newly implemented protest law as well as assaulting police officers, asked the court to document their assault, while their lawyers walked out of the session in protest against their mistreatment.

The court has ordered the prosecution to investigate the alleged assault.

"We look to the Egyptian government to ensure the safety of all those arrested or detained ... and [ensure they] are afforded due process and fair and transparent trials," Psaki added.

The UN Human Rights Council issued a joint statement last Friday with 27 other countries expressing concern over what it described as restrictions of "rights to peaceful assembly, expression and association, and about the disproportionate use of lethal force by security forces against demonstrators which resulted in large numbers of deaths and injuries."

The UN's statement mirrors the US's annual Country Reports on Human Rights Practices for Egypt in 2013, released in late February. The report condemned the "removal of an elected civilian government," in reference to the ouster of president Mohamed Morsi on 3 July 2013 following mass protests against his rule. It also criticised security forces' mistreatment of protesters.

In response to the US's report, Egypt's foreign ministry spokesman Badr Abdel-Ati denounced what he described as the US "appointing itself a lawyer and an advocate for human rights issues" in the world without a "legitimate basis."