KIRKUK, Iraq, May 17 — Ethnic tensions between Kurds and Arabs exploded into violence in this northern city today, as clashes in several neighborhoods left at least 5 dead. It was the worst violence in this city since the war.

Pickup trucks carrying armed men drove into Kirkuk from the town of Hawija, witnesses said. Kurdish witnesses to the violence said the armed men were Arabs, who were shouting slogans of support for Saddam Hussein. Gunfights ensued in two neighborhoods, said people who had been wounded in the fighting.

By late afternoon, it remained impossible to determine who was responsible for the shooting. The wounded seemed to be predominantly Kurdish, though the dead were split almost evenly between Arabs and Kurds.

The violence began two days ago, when Kurds harassed Arabs in an outdoor market and a bridge called Asho-Hada. Rouad Aziz, a resident of Qadesiya, said he had been beaten and threatened by Kurds on Friday. The Kurdish police said Arabs had cut the throats of four Kurds in another neighborhood on Thursday. The body of a man who had been decapitated was in the city morgue today.

"Yesterday Arabs were brought in with stab wounds, and today patients have been mostly Kurds," said Dr. Akhmed Makhmud, the head doctor on duty at the Republican Hospital. He said 20 patients were being treated for gun wounds, one in intensive care. His hospital had to send for extra medicine and surgical materials yesterday, he said.

As American forces try to keep order here, complicated ethnic tensions could prove to be a pitfall. American soldiers interviewed today said that sorting out the truth was difficult.

"It's tribal fights," said Sgt. Christopher Choay, of the 173rd Airborne Brigade. "It's hard for us to tell who is who. We can't take anyone's side. We're like a messenger caught in the middle."

An American military spokeswoman here said military patrols were "out assessing the situation," but gave no further details on the shootings.

In all, 40 people were wounded, mostly in gunfire, said doctors from the city's two main hospitals. At least three are in intensive care and are not likely to live through the night, said Muhammad Abdul Kaha Shakir, an emergency-ward doctor at Saddam Hospital, where most of the wounded were taken.

It was unclear today the extent to which the violence was a sign of things to come. Tension between Arabs and Kurds has been at a low boil since several flare-ups just after the war. This was the most deadly incident since then.

Kurds say they are fed up with what they call a "soft approach" by American forces with the Arabs, who were resettled here by Saddam Hussein during a brutal campaign against the Kurds in the late 1980's.

"Every day for 12 years Kurdish people see graves, bones and skulls," said Dr. Ahmed Majid, treating victims at Saddam Hospital. "They want to take revenge. They are saying: `Why are the Americans acting so slowly? Why can't we go back to our homes?' "

The Arabs, for their part, say they are afraid for their lives as armed Kurds shoot at their neighborhoods and intimidate residents during nighttime patrols. Automatic gunfire rattled windows in the Qadesiya neighborhood at midday today, a largely Arab area that borders a Kurdish quarter.

In the Kurdish quarter across the street, groups of men with guns patrolled many street corners. They said they were protecting their blocks from Arabs, who, they contended, had been driving through their neighborhood, shooting automatic guns.

Some residents lashed out at American forces, saying they had failed to provide the security needed for peaceful streets. In the Iskan neighborhood, American soldiers were shooting, said people who had been wounded in the violence. A police officer lying in the hospital ward said American troops had mistakenly fired on him. The military spokeswoman, Josslyn Aberle, said she had no details of American forces' activities in the neighborhood.

"We welcomed the Americans with flowers," said Kalid Arif, 61, whose son was in the emergency ward with a bullet wound that Mr. Arif said came from American soldiers. "Now this. You came to liberate and we are dying. My son will die for nothing."

At noon today, the hospital was a maze of frantic relatives and men on stretchers with bullet wounds, doctors stooping over them, sweating in the heat. A boy of 14 with a serious head injury called out to parents who did not yet know he was there.

After a day of accusations, Dr. Shakir of the Saddam Hospital made an assessment. "Shooting — they are all shooting," he said, looking at the intensive-care patients. "Everyone has a gun — Kurd and Arab — and everyone is ready to use it."