The deaths were announced as Iraqi fighters blew up a pipe supplying water to the capital while a new resistance group told Aljazeera it was dedicated to the country’s liberation.
The attack took place on Abu Ghuraib prison on the outskirts of Baghdad, the US military said on Sunday.
A US Army spokesman told journalists three mortar rounds hit the jail the day before, resulting in the immediate death of three prisoners and another three later in hospital.
The spokesman would not comment on who was responsible for the attack or why the prison was targeted, only saying the incident was under investigation.
Water supply disrupted
In a separate incident, damage to a water pipeline on Sunday caused huge floods in the eastern quarter of the capital and deprived an estimated 300,000 people of running water, the International Committee of the Red Cross said.
Large parts of Baghdad are now without electricity and water.
ICRC spokeswoman Nada Dumani said a rocket-propelled grenade had hit an open-air section of a pipeline linking the Sabah Missan pumping station with the eastern Baghdad district of Rasafa.
Technicians from Baghdad's water company have isolated the damaged part of the pipe and hope to have finished repairs by Tuesday, Dumani told AFP.
New resistance group
Meanwhile, a new resistance organisation has declared its intention to fight US-led occupation forces until they are driven out of Iraq.
In an exclusive Aljazeera broadcast, one of seven armed men read out objectives of the Iraqi Islamic Resistance Movement.
They emphasised the group was not formed as a reaction to US mistreatment of Iraqi citizens but as a movement to restore the country’s national sovereignty.
The spokesman added US-led occupation forces were not giving accurate figures for their dead and wounded, claiming at least 300 soldiers are killed each month.
Resistance to foreign troops in Iraq has increased substantially, with one attack near Basra resulting in Denmark’s first casualty since the occupation began.