Iraqi tanks and helicopters are being sent to the northern city of Mosul and troop reinforcements should arrive later on Sunday for a big offensive against al Qaeda militants, Iraqi security officials said.

Prime Minister Nuri al-Maliki said on Friday that Iraqi forces were preparing for a 'decisive' offensive against al Qaeda in Iraq to push the Sunni Islamist militants out of their last major urban stronghold.

US military officials on Sunday said their own operations around Iraq's third largest city were continuing.

'As it stands ... we are executing day-to-day operations in support of Operation Phantom Phoenix,' said Major Gary Dangerfield, spokesman for the 3d Armored Cavalry Regiment in Mosul, referring to a nationwide offensive launched this month.

'We are not in a position to validate the prime minister's future plans.'    

US military commanders say al Qaeda, blamed for most serious bombings in Iraq, has regrouped in northern provinces after being squeezed out of the western province of Anbar and from around Baghdad during security crackdowns last year.

Military spokesman Rear Admiral Greg Smith said al Qaeda had used two 15-year-old boys to carry out suicide bombings in the past week, one in Mosul and the other in Tikrit.

'We're not sure if one of these children even knew he was being used to deliver a bomb,' Smith said.

'These attacks were perpetrated at a funeral, a solemn religious ceremony, and at a school, a place that should be a safe haven for the young,' Smith told a news conference.

Major-General Riyadh Jalal Tawfiq, commander of military operations in Nineveh province, said additional Iraqi troops would arrive in Mosul later on Sunday from Baghdad, with more expected in the days after that. He gave no details of numbers.

The US military has some 3,000 troops in and around Mosul, capital of Nineveh province.

Maliki made his announcement after a blast blamed on al Qaeda killed 40 people and wounded 220 in Mosul on Wednesday. The explosion was in an unoccupied building that officials said was used by al Qaeda to store weapons and explosives.

Defence Ministry spokesman Mohammed al-Askari said tanks, armoured vehicles and helicopters were being sent to Mosul, 390 km (240 miles) north of Baghdad, for an offensive 'very soon'.

Askari said Defence Minister General Abdel Qader Jassim had visited Mosul to meet military commanders. The Interior Ministry said the Mosul push would include 3,000 extra police.

The US military calls al Qaeda in Iraq the biggest threat to Iraq's security. Despite frequent violence in northern Iraq, overall attacks have fallen sharply across the country, with the number of attacks down 60 percent since last June.

That has been attributed to an extra 30,000 US troops sent to Iraq last year, the growth of mainly Sunni Arab neighbourhood security units and a ceasefire by the feared Mehdi Army militia of Shi'ite cleric Moqtada al-Sadr. - Reuters