BAGHDAD, Iraq – In a well-publicized show of force,American and Iraqi forces swept into the countryside north of the capital in 50 helicopters yesterday looking for insurgents in what the American military called its “largest air assault” in nearly three years.

There was no bombing or firing from the air in the offensive northeast of Samarra, a town 60 miles north of Baghdad, the American military said. All 50 aircraft were helicopters – Black Hawks, Apaches, and Chinooks – used to ferry in and provide cover for the 1,450 Iraqi and American troops.

The military said the assault – Operation Swarmer – aimed to clear “a suspected insurgent operating area” and would continue over several days.

Residents in the area of the assault reported a heavy American and Iraqi troop presence and said large explosions could be heard in the distance. American forces routinely blow up structures they suspect as insurgent safe-houses or weapons depots. It was not known if they met any resistance, but the military reported detaining 41 people.

The attack was launched as Iraq’s new parliament met briefly for the first time. Lawmakers took the oath but did no business and adjourned after just 40 minutes, unable to agree on a speaker, let alone a prime minister.

Still, the session marked a small step toward forming a unity government that the Bush administration hopes will calm the insurgency and enable it to begin withdrawing U.S. troops.

Operation Swarmer also came as the Bush administration was attempting to show critics at home and abroad that it is dealing effectively with Iraq’s insurgency and increasingly sectarian violence.

A White House spokesman, Scott McClellan, denied it was tied to the new campaign to change war opinion. “This was a decision made by our commanders,” he said, adding that President Bush was briefed but did not specifically authorize the operation.

The American military forces have been trying to build up the Iraqi army so that it can play a leading role in fighting the insurgents.

The operation appeared concentrated near four villages – Jillam, Mamlaha, Banat Hassan, and Bukaddou – about 20 miles north of Samarra. The settlements are near the highway leading from Samarra to the city of Adwar, scene of repeated insurgent roadblocks and ambushes.

“Gunmen exist in this area, killing and kidnapping policemen, soldiers and civilians,” a spokesman for provincial government’s joint coordination center in nearby Dowr, Waqas al-Juwanya, said.

A Pentagon spokesman, Lieutenant Colonel Barry Venable, said the operation was the biggest air assault since April 22, 2003, when the 101st Airborne Division launched an operation against the northern city of Mosul from Iskandariyah, south of Baghdad.

Many operations in Iraq since then – in such cities as Fallujah, Ramadi, and Najaf – have included far more troops. But none has involved such a large force moved in by air. About 650 American and 800 Iraqi troops were participating yesterday.

The Pentagon said there were no reporters embedded with American troops, and it released video and a series of photos of preparations for the as sault. The images showed soldiers receiving a preflight briefing from a UH-60 Blackhawk crew chief, soldiers and aircraft positioned on an airstrip, and helicopters taking off over a dusty landscape.

But chief of the U.S. Central Command, General John Abizaid, sought to downplay the uniqueness of the raid.

“I wouldn’t characterize this as being anything that’s a big departure from normal or from the need to prosecute a target that we think was lucrative enough to commit this much force to go get,” General Abizaid said.

In recent months American forces have routinely used helicopters to insert troops during operations against insurgent strongholds, especially in the Euphrates River valley between Baghdad and the Syrian border.

The largest city near the operation, Samarra, was the site of a massive bombing against a Shiite shrine on February 22 that touched off sectarian bloodshed that has killed more than 500 and injured hundreds more.

It is a key city in a major part of the so-called Sunni triangle where insurgents have been active since shortly after the American-led invasion three years ago, Salahuddin province. Saddam Hussein was captured in the province, not far from its capital and his hometown,Tikrit.

A presidential security adviser, Lieutenant General Wafiq al-Samaraei, said the operation was targeting “a bunch of strange criminals who came from outside the country and among them a bunch of Iraqi criminals who help them.”

Iraq’s interim foreign minister, Hoshyar Zebari, said the attack was necessary to prevent insurgents from forming a new stronghold such as they established in Fallujah, west of Baghdad, until they were flushed out by American forces at the end of 2004.

“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,” Mr. Zebari said on CNN.