BAGHDAD -- US and Iraqi forces began a major helicopter and ground attack yesterday on an insurgent stronghold near Samarra, a city dominated by Sunni Arabs, where the bombing last month of a Shi'ite shrine led to sectarian bloodshed.
The assault took place 80 miles north of Baghdad as the parliament, elected three months ago, held its inaugural session in the capital. The meeting was adjourned so that political leaders could resume US-guided talks on the makeup of a new government's leadership.
The joint military operation and the new parliament are elements of a US strategy to start bringing home troops, who arrived almost three years ago to topple President Saddam Hussein. Iraq's military has been taking a bigger role in attacks on a Sunni Arab-led insurgency made up in part of Hussein supporters.
And under US pressure, leaders of all parliamentary factions are trying to avert full-scale sectarian conflict by holding talks aimed at bringing Sunni representatives into a broad coalition.
Adnan Pachachi, at 83 the oldest member of the new parliament, underscored the urgency of the task in unusually blunt remarks to his colleagues after he had been appointed temporary speaker.
''The country is going through dangerous times . . . and the perils come from every direction," he said at the nationally televised session. ''We have to prove to the world that there will not be civil war among our people. The danger is still there, and our enemies are ready for us."
''We're still at the beginning of the road to democracy," he added, ''and we're stumbling."
In announcing the counter-insurgency assault, called Operation Swarmer, US officials emphasized the involvement of Iraqi's army, which provided 800 of the 1,500 troops involved.
That is fewer total troops than have taken part in assaults to drive insurgents from Fallujah, Ramadi and other cities. But more than 50 aircraft, mainly helicopters, helped transport the troops, making it the largest airborne attack in Iraq since April 2003, military officials said.
A statement by the US command said that the raids by the 101st Airborne Division and Iraq's First Brigade would continue, and that a number of insurgent weapons caches, containing artillery shells, explosives, army uniforms, and materials for making car bombs, had been discovered.
Lieutenant Colonel Edward S. Loomis, a US military spokesman, said 40 people were detained. There were no reports of resistance or casualties.
Residents of the area, northeast of Samarra, said they had heard explosions after troops swooped in after 7 a.m.
They said the operation was concentrated around four villages that have harbored followers of a Jordanian, Abu Musab al-Zarqawi, whose Al Qaeda in Iraq organization has been accused of the mosque bombing on Feb. 22.
Repeated sweeps by US soldiers have failed to secure the Samarra area. US and Iraqi officials said the timing of the latest raid was unrelated to the mosque bombing or the third anniversary next week of the US-led invasion.
Hoshyar Zebari, Iraq's interim foreign minister, told CNN that the attack was aimed at preventing insurgents from creating a stronghold. Zebari referred to insurgent centers such as the ones they had set up in Fallujah for much of 2004, and later along the Euphrates River in western Iraq.
''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," Zebari said. ''They have to be pulled out by the roots."
In Baghdad, Iraqi officials imposed a daylong vehicle ban in an effort to help protect the newly elected legislators.
The members of the new legislature gathered inside the fortified Green Zone, protected by barriers and concertina wire.
Two mortars fired from outside the Green Zone fell harmlessly near the convention center after the legislators had left.
The 275-member Council of Representatives is Iraq's first democratically chosen parliament in half a century.
But with no agreement yet among political factions over the makeup of the country's leadership, the new parliament cannot elect its own officials or conduct substantive business.
In other developments, at least 20 people were killed or found dead yesterday.
Three of them were shot to death in the usually quiet Kurdish region in the north during clashes between police and an angry crowd protesting shortages of electricity and water.