Thirty-five prisoners facing terrorism charges escaped through a sewage pipe from a temporary jail in Iraq's restive northern city of Mosul on Thursday, police and security officials said.
Police recaptured 21 of the escapees and imposed a curfew on the city as they looked for the rest, officials said.
Mosul, an Al-Qaeda stronghold located 390 km north of Baghdad in the troubled province of Nineveh, has seen a number of big prison breaks. Last year 23 prisoners convicted on terrorism charges escaped from the city's Ghazlani prison.
In 2007, dozens of Al-Qaeda led militants stormed Badoush prison just outside Mosul and freed up to 140 prisoners. In December 2006, a nephew of Iraqi dictator Saddam Hussein escaped the same prison.
"This is the third incident taking place in Nineveh province of prisoners escaping. There are dangerous leaders in the prisons, who need more intense attention from the security forces," said Abdul-Raheem al-Shimeri, the head of the security committee in the Nineveh provincial council.
Violence has dropped dramatically in Iraq since the height of sectarian warfare in 2006-07 but shootings, bombings and other attacks happen daily.
Mosul is considered Iraq's last urban stronghold of Sunni Islamist Al-Qaeda militants and much of the funding for the group's attacks across the country is believed to come from Nineveh.
More than eight years after the United States ousted Saddam Hussein, Iraq is still building its police force and army to battle Sunni insurgents and Shiite Muslim militias in the country, as well as defending against external threats.
US forces are preparing to withdraw from Iraq by the end of the year, according to a bilateral security agreement.