BAGHDAD, Iraq, April 17 — American and Iraqi troops sealed off one of Baghdad's most prominent Sunni Arab neighborhoods on Monday after a night of raging gun battles that left homes and storefronts riddled with bullets and at least one civilian dead, Iraqi officials and witnesses said.

The closing of Adhamiya, in northern Baghdad, seemed to signal deteriorating security in a neighborhood where attacks on American and Iraqi forces had ebbed in recent months. The area is home to hard-line Sunni Arabs who remain hostile to the Shiite-led government and the American presence. At its center is the well-known Abu Hanifa Mosque, where Saddam Hussein made his final public appearance in April 2003 before fleeing Baghdad and the American invasion force.

A leading Sunni Arab political group, the Iraqi Islamic Party, released a statement on Monday calling for calm and saying that a "human disaster might occur." It said the clashes were between Iraqi government forces and residents of Adhamiya, implying that the uniformed forces were the aggressors.

At least one civilian was killed and five injured in the battles, an Interior Ministry official said.

The origins of the violent confrontation remained murky. A guard at Abu Hanifa Mosque, who gave his name as Abu Aus, said in a telephone interview that fighting erupted around midnight when gunmen clashed with Iraqi soldiers at Al Sabah library nearby. Police commandos, long suspected of operating Shiite death squads, then entered the area and joined forces with the soldiers, he said.

The violence damaged homes, shops and cars for blocks around. Bullets crashed through windows, people dived for cover under their beds, and some residents scrambled up to their rooftops and opened fire with Kalashnikovs.

The shooting quieted down Monday morning only after American troops and Iraqi soldiers rolled through the streets with armor, Abu Aus said. He added that at least 5 civilians were killed and 14 injured, but there was no official confirmation of those numbers.

A spokesman for the American military said Monday evening that American troops had been active in Adhamiya but declined to give details. An Iraqi Army commander said on Al Iraqiya, the state-run television network, that Iraqi soldiers had fought insurgents in the streets.

Because of its prominent history and the renown of Abu Hanifa Mosque, Adhamiya is one of the most volatile Sunni Arab neighborhoods in Baghdad. Assaults there against American troops were common from the start of the insurgency and ballooned into full-scale street fighting during uprisings in 2004.

Last year, though, attacks fell, and it appeared that American and Iraqi forces had been able to clamp down somewhat on the guerrillas. But the violence in Adhamiya seemed only to migrate to many neighborhoods in western Baghdad, closer to the airport and Abu Ghraib prison.

The recent street fighting shows that Adhamiya is still prone to large-scale violence. After the bombing of a revered Shiite shrine in Samarra on Feb. 22, when Shiite militiamen began a deadly rampage against Sunnis Arabs in Baghdad, residents of Adhamiya formed neighborhood militias and barricaded streets leading to the area. Men patrolled with Kalashnikovs, wary of any incursion by the Shiite-led Iraqi police or the Mahdi Army, a Shiite militia.

The violence was not confined to Adhamiya; policemen found a dozen bodies in three different areas of Baghdad on Monday, all shot in the head, the Interior Ministry official said. An employee at Yarmouk Hospital said the police brought 18 bodies to the hospital on Monday. It was unclear whether some or all of those were the same as those reported found by the Interior Ministry.

The police also found the body of the brother of Saleh al-Mutlak, a conservative Sunni Arab legislator who won widespread support in hard-line Sunni Arab regions in last December's elections. Mr. Mutlak's brother was kidnapped weeks ago. Last week, gunmen killed the brother of Tariq al-Hashemi, another prominent Sunni Arab legislator.

The lengthy stalemate in talks to form a new government appeared no closer to resolution on Monday. Shiite leaders opposed to Prime Minister Ibrahim al-Jaafari's effort to keep his job agreed Sunday that his party could nominate the next prime minister, as long as Mr. Jaafari stepped down. But on Monday, the two deputies to Mr. Jaafari who are considered front-runners for the post said publicly that the party still supported Mr. Jaafari.

The Islamic Dawa Party "cannot present any candidate unless Jaafari decides to step aside," a deputy, Ali al-Adeeb, told The Associated Press. "So far, his position has not changed."

The trial of Saddam Hussein resumed Monday, with handwriting experts confirming the authenticity of his signature on a document stemming from a government crackdown on Shiites in the 1980's. He and seven co-defendants are accused of torturing and executing 148 men and boys from Dujail, a Shiite village, after a failed assassination attempt on Mr. Hussein in 1982. The judge adjourned the trial until Wednesday to give the handwriting experts more time to peruse the documents.

In the morning, a concealed bomb in central Baghdad killed at least one civilian and wounded three as an Iraqi Army convoy rolled by. Gunmen kidnapped a doctor and six electrical engineers in two incidents, and 30 workers at a trading company were abducted Sunday, police officials said. In Kirkuk, in the north, a policeman was killed and two were wounded by a bomb, while a civilian was killed and three were wounded in a clash between insurgents and military forces in a nearby village.

Ali Adeeb and Omar al-Neami contributed reporting from Baghdad for this article, and an Iraqi employee of The New York Times from Kirkuk.