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 before 21 were recaptured, officials said.

Mosul, an al Qaeda stronghold located 390 km (240 miles) 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.

With help from U.S. military aerial surveillance, Iraqi police managed to apprehend 21 of the latest 35 escapees and imposed a curfew on the city as they looked for the rest, officials said. A U.S. military official acknowledged the remaining fugitives could remain at large for some time.

The reality is a few will not be captured in the immediate future, said Colonel Brian Winski, commander of 4th Advise and Assist Brigade, 1st Cavalry Division, speaking by video-conference from Iraq to reporters at the Pentagon.

Winski described the prisoners as local Iraqi members of a militant cell, the kind that conducts attacks using roadside bombs and mortar fire. Still, he did not directly answer questions about whether they were linked to al Qaeda.

None of them were foreign fighters. None of them were high level leaders, he said.

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.

Winski, however, defended the Iraqi response and stressed that the facility was not a proper prison, and just a temporary detention centre. He said the Iraqis would catch the fugitives in time.

I'm quite confident, again with some assistance from us, that they will be able to find them, he said. They know who (the fugitives) are so they will find them and regain custody and control of them eventually.

Iraqi intelligence was focussing on where the escaped prisoners would likely go, and readying operations to catch them when they surface, Winski said.

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 Shi'ite Muslim militias in the country, as well as defending against external threats.

U.S. forces are preparing to withdraw from Iraq by the end of the year, according to a bilateral security agreement.

Al-Qaeda Iraq Terrorism Iraq