After spending the night in abandoned homes, the more than 5,000 U.S. and Iraqi troops who had swept into the northern city of Tall Afar awoke Saturday morning to broadcasts from mosques calling residents to fight the invasion.

But the troops met little resistance as they continued raiding houses Saturday to gather information about the insurgents who have controlled large parts of the city for nearly a year.

In one of the few pockets of fighting, insurgents fired seven rocket-propelled grenades at U.S. tanks from adjacent buildings in the western neighborhood of Qadisiyah. A U.S. jet destroyed much of the block with a 500-pound satellite-guided bomb, commanders said. Soldiers also destroyed at least half a dozen roadside bombs and discovered a large cache of artillery rounds hidden in one of the many lush valleys that divide the city.

For the second consecutive day, U.S. forces reported no casualties.

"We expected them to fight back more than they did today, especially given some of the neighborhoods we were moving through," said Capt. Alan Blackburn, 30, of Mooresville, Ind., commander of Eagle Troop, 2nd Squadron of the Army's 3rd Armored Cavalry Regiment, which is leading the assault.

Blackburn said the estimated 300 to 500 insurgents believed to be operating in Tall Afar appeared to be massing in the restive neighborhood of Sarai, east of downtown, where U.S. patrols are frequently attacked.

The operation in Tall Afar, considered a transit point and logistics hub for insurgents operating across northern Iraq, is the largest urban invasion in Iraq since the U.S. siege of Fallujah last November. U.S. troops led an offensive in Tall Afar, a city of more than 200,000 people about 40 miles from the Syrian border, one year ago this month, but largely withdrew soon after. Insurgents quickly returned to the city.

Soldiers from Blue Platoon, stationed in the northeastern neighborhood of Qadra, were awake hours before dawn, cleaning their weapons with steel-bristled brushes and WD-40.

"We'll be going into a considerably rougher area than we did yesterday, so we have to stay alert," Capt. Noah Hanners, the platoon commander, told his soldiers.

The soldiers raided dozens of homes, using sledgehammers and occasionally employing explosive charges to knock down doors. They confiscated AK-47 assault rifles and a set of rusted brass knuckles.

The soldiers also interviewed residents for information about insurgent activity, but a shortage of interpreters complicated their efforts. The platoon, divided into two sections, shared a lone interpreter throughout the day.

Abbas Amin Abbas, a Tall Afar physician, told the soldiers he had shifted his dermatology practice from a nearby hospital because of recent clashes between insurgents and the army.

"Why should I be between you and the bullets?" he asked the soldiers. "When we see you coming, we stay inside. We don't want to expose ourselves to the danger."

In other parts of the country, at least 17 Iraqi soldiers and four civilians were killed in insurgent attacks. Much of the violence was concentrated in the northern province of Diyala, where gunmen attacked two checkpoints manned by Iraqi police and soldiers. A total of 13 soldiers were killed, hospital officials said.

Four Iraqi soldiers were killed in an ambush 60 miles south of the northern city of Kirkuk, police said.

Four civilians were killed in the northern city of Samarra when mortar rounds fired at a U.S. military installation hit a residential neighborhood, the Associated Press reported.

Near Kirkuk, armed men set fire to oil leaking from a major pipeline, according to Mahmood Abdullah, a member of the government's pipeline protection force. The pipeline, which carries crude oil from Kirkuk to the Turkish city of Ceyhan, eventually caught fire.

The Reuters news service reported that a bomb, rather than arson, caused the pipeline fire and that the blaze shut down exports to Ceyhan on the Mediterranean.

The Kirkuk-Ceyhan pipeline is capable of exporting roughly 1.5 million barrels per day, although output has averaged around 200,000 barrels per day since the U.S.-led invasion began, because of attacks and sabotage.

Special correspondent Hassan Shammari in Baqubah contributed to this report.