BAGHDAD: A group of al Qaeda prisoners was caught trying to tunnel out of Iraq's Abu Ghraib prison, an official said on Sunday, after the militants said they would step up their fight against the government.

Iraq's Shi'ite-led government is concerned that Sunni Muslim militants will get a boost from the insurgency in neighbouring Syria, where mainly Sunni rebels are battling President Bashar al-Assad.

Jail breaks are common in Iraq and security at prisons was beefed up this week after five militants stormed a police counter-terrorism headquarters on Tuesday in an attempt to free al Qaeda prisoners. All five were killed in a long gun battle.

A spokesman for the justice ministry, Haider al-Saadi, said in a statement that 11 "dangerous prisoners" at Abu Ghraib dug down three metres and had tunnelled along 20 metres using a frying pan and part of a ceiling fan before they were discovered.

They had fashioned breathing apparatus from soft-drink cans stuck end to end.

Another official in the ministry told Reuters that the men were al Qaeda members. The high security prison, notorious for abuse both during the rule of Saddam Hussein and under U.S. occupation, houses several thousand convicted militants.

"The late shift guard around midnight on Saturday heard some banging and digging under the concrete," Saadi said. "The guards then conducted an arrest operation ...in a professional way by filling the tunnel with water to force them out."

Al Qaeda's Iraqi wing, the Islamic State of Iraq (ISI), warned last month that it planned to revitalise its campaign, weakened after years of losses against U.S. troops and Iraqi allies.

Iraqi Sunni Muslim militants have travelled to Syria to support the rebels, security experts say, and the ISI is reaping funds and recruits and fighters are criss-crossing the border.

AIR SUPPORT FOR SYRIA BORDER

Sunni insurgents often attack Shi'ite targets in Iraq to try to reignite the sectarian violence that killed tens of thousands of people in 2006-2007, and topple the Shi'ite-led government of Iraqi Prime Minister Nuri al-Maliki.

Baghdad has sent troops and tanks to strengthen security at its 680-km (420-mile) frontier with Syria and a defence ministry spokesman told Reuters on Sunday that forces at the border have now been given air support.

"We have monitoring planes to supply the ground troops with pictures and information. We also have planes and helicopters that can support troops on the ground with fire," Colonel Dhiya al-Wakeel said.

Wakeel said Iraq was receiving no foreign assistance with protecting its border and airspace, a job its military have been responsible for since U.S. troops pulled out in December.