KIRKUK, Iraq (AP) -- While the world focuses on Baghdad's security, a series of bombings here may be the long-feared start of a second deadly war in Iraq -- this one between Kurds and Arabs, both with claims on a territory atop one of the world's largest oil reserves.

If the escalating violence in Kirkuk erupts into all-out fighting between heavily armed Kurdish and Arab groups, it could spark a wider conflict involving Turkey or Iran. That risk puts the United States in a bind, caught between ally Turkey, which is on the side of Arabs and ethnic Turkomen here, and the Kurds, another strong U.S. ally.

The issue is coming to a head because of a provision in the Iraqi constitution that calls for a referendum by year's end on Kirkuk's future. Arabs and Turkomen, backed by Turkey, want to put the vote off -- worried about Kurdish dominance and more violence if the referendum is held and Kurds win.

But Kurds are determined to press ahead. They deny it's because of the black gold in the ground.

"We will have Kirkuk -- not for its oil, but because it is our history," said Rizgar Ali Hamajan, a Kurd who is chief of the local provincial council.

In the past two weeks, the city 180 miles north of Baghdad has suffered a wave of bombings, including six car bombs on one day alone. One targeted a main Kurdish political organization. Another bomb this week seriously wounded a Kurdish teacher. Some Kurds claim that Sunni Arab groups with al-Qaida links are now operating here, but Turkomen and Arabs also have been hit by violence.

The dispute centers on whether this ancient city should become part of the semi-independent Kurdish zone in northeast Iraq, or remain as it is, part of broader Iraq, governed by the Arab-led coalition government in Baghdad. The referendum, whose date has not been agreed upon, would settle that by asking residents which they preferred.

Unlike in Baghdad, in Kirkuk there are sharp lines between the warring sides, a legacy of a battle for dominance here that predates the U.S.-led invasion of 2003.

On one side of the divided city are people like Abdul-Karim Wadi, a Shiite Arab, who got what amounted to thousands of dollars in cash and a free apartment to move to Kirkuk from Baghdad 18 years ago. He was part of Saddam Hussein's campaign to flood the city with Arabs and cleanse it of Kurds.

Now, Wadi says, Kirkuk is his home and he has no plans to leave. He says he had no idea about Saddam's intentions when he moved here.

On the other side are people like Soham Qadir, a plump Kurdish woman with a quick smile, who lives in a two-room house made of mud and stone on the city's northwest fringe. Driven out of Kirkuk in 1995 by Saddam's plan, she and her family returned in 2003 after the U.S.-led invasion -- encouraged to do so by Kurdish politicians.

"We have the right to Kirkuk. It belongs to the Kurds," Qadir said.

Chillingly, each side has increased its warnings that it is armed and ready to fight.

Kurds, in particular, have well-armed, highly trained peshmerga militias with years of experience fighting in the past conflicts of northern Iraq.

But Arabs too say they are ready to fight. "We tell the Kurdish political parties to have a clear understanding, that if they try to make Kirkuk a part of Iraqi Kurdistan, then war is coming here," warned Sheik Abdul Rahman Munshid, a Sunni Arab leader.

"They should know we are ready, we are already organized," said Munshid, speaking in his palatial white marble home hidden behind high walls. Munshid's neighborhood is known for its links to Saddam's loyalists and Sunni insurgents, some with al-Qaida links, according to residents.

A powerful ally of the Arabs are the Turkomen, a minority in Iraq concentrated in the north. They accuse Kurds of intimidation bombings and kidnappings against them. They say that by resettling their people, the Kurds are changing the city's ethnic balance and taking away Arabs' and Turkomen's voting rights.

"If Kirkuk goes to Kurdistan, we will fight. I will fight," warned Ali Mehdi Sadiq, a representative of the Turkomen. Such a war, he warns, "will bring in other countries in the region, Turkey and Iran. They care about what happens here."

American experts agree that the referendum carries high risks.

The U.S. Iraq Study Group, the panel led by former Secretary of State James A. Baker III, said in its report in December that "given the very dangerous situation in Kirkuk ... a referendum on the future of Kirkuk would be explosive and should be delayed."

So far, President Bush's administration has not supported canceling or delaying the vote.

Iraqi President Jalal Talabani, a Kurd, has warned Turkey against interference.

So far, Turkey has held its fire, despite what is says are frequent provocations. Turkey has been fighting a Kurdish independence movement within its borders and has faced harassing attacks by Kurdish guerrillas, aided by allies who cross the border from Iraq.

Turkey and Iran also fear an economic boom in Iraq's Kurdish region. Should Iraqi Kurds gain control over the Kirkuk oil fields, it could embolden and finance the Kurds inside their own countries to push harder for autonomy. Kirkuk has six oil fields containing one of Iraq's largest oil reserves of about 8 billion barrels.

Both Iran and Turkey have sent additional troops to their borders this year, and fights between Kurdish guerrillas and Iranian security forces also are up.

There are no accurate figures of the numbers of Kurds to return to Kirkuk in the last three years, but estimates range as high as 300,000. Most believe Kurds are now a majority here.

The last ethnic-breakdown census in Iraq was conducted in 1957, well before Saddam began his program to move Arabs to Kirkuk. That count showed 178,000 Kurds, 48,000 Turkomen, 43,000 Arabs and 10,000 Assyrian-Chaldean Christians living in the city.

By Bassem Mroue And Kathy Gannon

Mroue reported from Baghdad and Gannon from Kirkuk. AP writer Yahya Barzanji in Kirkuk also contributed to this report.