Military spokesman Lieutenant Colonel Barry Johnson said eight leading detainees had been freed on Saturday.
He said they were among 27 prisoners, considered senior members of the administration overthrown by US forces in 2003, who now posed no threat to security, were neither charged with crimes nor material witnesses and had no intelligence value.
Johnson declined to identify any of the detainees or comment on the fate of those still held prisoner.
Baghdad lawyer Badia Arif said 26 people, including five who were ill, were in the process of being released. He said among them were Huda Ammash, nicknamed "Mrs Anthrax" by the popular press in the West, and Rihab Taha, dubbed "Dr Germ".
"The list included Huda Ammash and Rihab Taha," Arif said, declining to say whether they had left Baghdad airport, where they were held along with others.
"There were no accusations against them. Other lists are being prepared and might include Tareq Aziz," said Arif, who acts as an attorney for Aziz and Ammash.
Ammash and Taha were both detained in May 2003, shortly after the overthrow of Saddam.
Taha, a British-educated biological weapons expert, was known for her role in making bio-weapons in the 1980s. Ammash, a former top Baath party official, was a biotech researcher.
Lawyers acting for Ammash have said she is gravely ill with cancer. She was the only woman included in the US military's list of the 55 most-wanted members of Saddam's regime and was the Five of Hearts in a deck of cards issued to help US soldiers identify fugitives.
Huda Ammash is the only woman
in the US most-wanted list
She has a master's degree from Texas Women's University and a doctorate in microbiology from the University of Missouri.
Taha was not on the US most-wanted list but was described by American officials as a former director of the Iraqi bacterial and biological warfare programme.
She has admitted producing germ warfare agents but said all such weapons were destroyed long before the US invasion.
Taha has a doctorate in plant toxins from the University of East Anglia in Britain, and is married to Amir Muhammad Rashid, a former Iraqi oil minister also in US detention.
Doctors for Iraq plea
A humanitarian group called Doctors for Iraq has called for an investigation into the torture of prisoners by Iraqi government forces and police.
In a statement, the group said it had seen evidence that torture had been widely used against civilians who were arrested without legal procedure or charge and transferred to secret prisons.
Aljazeera has aired footage
of tortured Iraqi prisoners
The prisons, which were closed to human rights organisations, are in Baghdad and other parts of the country, such as Mosul, Qut and Basra, said the group.
"It is unclear how many of these prisons exist and how many prisoners are being held," the group said.
Prisoners are subjected to forms of torture including sleep deprivation, being hung by their legs and arms for long periods, electrical shock and sexual abuse, the organisation said.
"Doctors for Iraq has received reports of rape being used as a tool of torture in some cases," a news statement read.
Women are being held and abuses carried out against them, said the group.
Doctors for Iraq says it is calling on the European Union, which is training Iraqi police forces, and the UN to uphold the rights of Iraqi citizens and to play a role in an international investigation into the alleged crimes.
Aljazeera last week aired footage of prisoners who showed evidence of torture in Iraqi jails.