Bomb-making equipment and military uniforms have been seized in a major air and ground assault on insurgent bases across Iraq's notorious Sunni triangle, US military sources reported this evening.

Hidden caches of artillery shells and explosives were also uncovered as Iraqi and American troops descended on isolated hideouts in the hilly Salahuddin province, between Tikrit and Samarra about 80 miles north of Baghdad.

The Pentagon described the offensive, codenamed Operation Swarmer, as the largest air assault since the 2003 invasion, whose third anniversary is just days away. It involved around 1,500 Iraqi and coalition troops, backed by 200 tanks and support vehicles and more than 50 aircraft, mainly UH-60 Black Hawk helicopters.

A senior Iraqi army officer told the AFP news agency that the intention was to flush out rebels loyal to Abu Musab al-Zarqawi, the leader of al-Qaeda in Iraq.

Black and white footage of the operation released by the Department of Defence in Washington showed soldiers searching brick-built huts. There were also images of a number of men being marched outside, their hands bound behind their backs.

Locals described large explosions near the villages of Jillam, Mamlaha, Banat Hassan and Bukaddou, about 18 miles north of Samarra. The villages are near a main road leading from Samarra to the city of Adwar.

A military spokesman said that the mission, intended to "clear a suspected insurgent operating area", had been launched this morning and was due to last for several days.

Hoshyar Zebari, the Iraqi interim foreign minister, said the attack had been necessary to prevent insurgents from forming a new stronghold such as they had established in Fallujah, west of Baghdad.

"After Fallujah and some of the operations carried out successfully in the Euphrates and Syrian border many of the insurgents moved to areas nearer to Baghdad," he told CNN. "They have to be pulled out by the roots."

Some commentators suggested that the US military may be hoping to break the cycle of violence and tackle the last remaining strongholds of the insurgency as they prepare to withdraw their troops.

Waqas al-Juwanya, a spokesman for Iraq’s joint coordination centre in nearby Dowr, said "unknown gunmen exist in this area, killing and kidnapping policemen, soldiers and civilians."

The assault was launched just hours before Iraq’s new parliament was sworn in behind the concrete blast walls of Baghdad's heavily fortified Green Zone with parties still deadlocked over the next government, vehicles banned from Baghdad’s streets to prevent car bombings and the country under the shadow of a feared civil war. The session lasted for just 40 minutes before being adjourned indefinitely in deadlock and confusion.

The Salahuddin province is a major part of the so-called Sunni triangle where insurgents have been active since shortly after the invasion. Saddam Hussein was captured in the province, not far from its capital, Tikrit.

On February 22, an important Shia shrine was bombed in Samarra unleashing the most fierce wave of sectarian violence since the invasion began.

It said the attack began with soldiers from the Iraqi Army’s 1st Brigade, 4th Division, the 101st Airborne Division’s 3rd Brigade Combat Team and the 101st Combat Aviation Brigade conducting a combined air and ground assault.

There was no figure available on the number of military or civilian casualties.

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