Soldiers from Iraq's 1st Brigade, 3rd Army Division and the US 2nd Squadron, 3rd Armored Cavalry Regiment pressed on with their offensive against insurgents in the northern town of Tal Afar, west of Mosul, the US military said late Tuesday.

"Major clashes occurred in the area of Tal Afar where Iraqi and coalition forces were successful in security operations," it said in a statement.

It said that troops had found and destroyed nine weapons caches and detained 73 suspects since the start of the operation on May 26.

Predominantly Turkmen Tal Afar, 100 kilometres from the Syrian border, is suspected by US and Iraqi officials of being a sanctuary for Sunni Arab foreign fighters infiltrating through the country's porous borders with its western neighbour and carrying out attacks in the area which includes Mosul.

It was the scene of major street fighting in September of last year that left dozens dead and has since then seen frequent attacks against US and Iraqi forces.

Since November, Mosul, Iraq's third largest city, has become a new front for the fight against insurgents with both US and Iraqi officials frequently announcing the arrest of senior aides to Iraq's most wanted man Abu Musab al-Zarqawi, the alleged frontman for the al-Qaeda terror network in the violence-ravaged country.

The US military said the operation in Tal Afar was part of 30 other operations involving both US and Iraqi troops or just Iraqi forces.

As the Iraqi government touted the success of Operation Lightning in Baghdad saying it would spell the "slow death" of the insurgency, rebels struck mostly further north of the capital on Tuesday in what has become a familiar pattern in a cat-and-mouse game that sees insurgents refocusing their attacks from one area of the country when they are under pressure to another.

Three US soldiers were killed in two separate attacks late Tuesday north of Baghdad, the military said.

Two soldiers were killed in "an indirect fire" attack on their base in Tikrit, while another was killed in a roadside bomb explosion during a patrol in the restive town of Balad.

The US casualties capped a bloody day that saw at least 33 Iraqis killed, making it one of the most violent days since May, during which about 700 Iraqis died in a frenzy of car bombs.

The worst attack was in the restive Sunni Arab town of Al-Hawijah, north of Baghdad, where 14 Iraqis were killed, half of them Iraqi soldiers, and 20 wounded in a trio of suicide car bombs against army checkpoint.

Nine people were killed in Mosul, including four peshmerga militiamen reportedly shot dead by police after they were mistaken for insurgents and three students killed when unknown gunmen burst into their apartment.

In the capital, an employee of the foreign ministry was killed in a drive-by shooting and a policeman was shot dead in the western Amil neighbourhood.

Government spokesman Leith Kubba said Operation Lightning, now expanded south of the capital to the so-called triangle of death, had netted 887 detainees since May 22, revised from an earlier figure of more than 1,200.

"Fighting these criminal networks ... and eradicating them will not happen with a knockout blow, but rather it will be a slow death and it will happen with continuous efforts to isolate them." Kubba said.