KIRKUK, Iraq — Iraqi state television reported early Monday that Iraqi forces had begun an assault on the Kurdish-held city Kirkuk and its oil fields, despite weeks of urgent efforts by the United States to keep tensions between its allies from boiling over into another war in the Middle East.
In a brief statement released to the state-run network, Iraq’s prime minister, Haider al-Abadi, ordered troops to impose security in the area “in cooperation with the people of Kirkuk and the pesh merga,” or Kurdish fighters.
State-run TV said the initial assault by Iraqi troops, counterterrorism forces and federal police did not encounter resistance as they sought to reclaim areas seized by Kurdish forces in 2014. But there were unconfirmed reports of clashes with the pesh merga, who maintain defensive lines around Kirkuk and the oil fields.
Military sources also reported exchanges of artillery fire, but those reports could not be confirmed.
The Iraqi military operation would be the first use of military force by the government in Baghdad in response to an independence vote last month by the autonomous Kurdish region in northern Iraq.
Both the Iraqi Army and the pesh merga have been trained and equipped by the United States as part of the American-led coalition battling Islamic State militants in the country. But the other major players in the conflict — the Shiite militias that make up a considerable amount of Iraq’s fighting strength — have largely been trained and supported by Iran.
In Washington, the Pentagon urged “all actors” in the region to focus on battling Islamic State militants and to avoid provoking disputes among Iraqis, Reuters reported.
It was unclear whether American troops were in the area Monday morning. A spokesman for the coalition in Baghdad did not immediately respond to a request for comment. There was no immediate response from the American Embassy in Baghdad.
The United States has provided intelligence, special operations forces, weapons, airstrikes and artillery to Iraqi forces battling Islamic State militants in the area, and have similarly backed Kurdish forces in that fight.
Kurdish government leaders and military commanders had vowed to fight any attempt by Iraqi forces to reclaim control of the Kirkuk area, which was captured by Kurdish forces after Iraqi troops fled an assault by Islamic State militants in 2014.
Hemin Hawrami, a spokesman for the president of the Kurdish region, Massoud Barzani, wrote on Twitter that pesh merga soldiers had destroyed four government Humvees and had twice repelled a government attack in a district south of Kirkuk. Those statements could not be confirmed.
In one tweet, Mr. Hawrami accused Mr. Abadi of using military force “to settle political issues.”
Mr. Abadi had demanded that Kurdish leaders surrender control of Kirkuk, the oil fields and other disputed areas that fell under Kurdish control three years ago. He repeatedly said in recent days that his government had no plans to attack Kurdish forces defending the Kirkuk area. On Friday, Mr. Abadi tweeted that media reports of an imminent assault were “fake news.”
Kurdish leaders have said that Baghdad moved large number of troops to confrontation lines south of Kirkuk after the coalition drove Islamic State militants on Oct. 5 from their last major urban stronghold in Iraq: the city of Hawija, about 40 miles southwest of Kirkuk.
The independence vote, in a referendum held Sept. 25, strained relations not only between Kurdish authorities and Baghdad, but between the Kurds and the United States. The United States government had adamantly opposed the referendum, saying it would undermine the fight against Islamic State militants, foment ethnic divisions and create instability in Iraq.
Mr. Barzani rejected an American proposal last month to cancel the referendum and enter negotiations with Baghdad facilitated by the United States.
The referendum, which was nonbinding, was opposed by every country in the region except Israel. Kurdish authorities said the measure passed with a nearly 93 percent “yes” vote for independence.
On Sunday night, Mr. Abadi accused Kurdish leaders of bringing fighters from the Kurdistan Workers’ Party in Turkey into Kirkuk, calling it “a declaration of war against the rest of Iraq and its federal forces.” He provided no evidence. The United States and European Union list the group, which is known as P.K.K. and is active near the Turkish border, as a terrorist organization.
Najmaldin O. Karim, a Kurd who is governor of Kirkuk Province, said in response to Mr. Abadi’s statement: “That’s absolutely not true. I deny it.” Mr. Karim said a handful of Iraqi Kurds who sympathize with the P.K.K.’s goals had arrived in Kirkuk over the weekend. But he said that they were not P.K.K. members or fighters, and that they were not armed.
For his part, the president of the Kurdistan Regional Government, Masoud Barzani, offered late on Sunday to negotiate with Baghdad on Kirkuk and other issues, without conditions. Mr. Abadi has said he will not negotiate unless the Kurds annul the referendum results.
Tensions were already high on Saturday night after news reports said Iraqi forces had issued a 2 a.m. Sunday deadline for Kurdish forces to withdraw from contested areas or face unspecified consequences. Officials in Kirkuk called the reports false.
In the weeks before, the Iraqi government moved to squeeze the landlocked region, shutting down overseas flights to two international airports. Late Sunday, Iraq’s foreign ministry announced that Iran, at Iraq’s request, had closed its borders with Iraq near the Kurdish region.
Both sides were clinging to hardened positions.
On Sunday in Dibis, about 30 miles northwest of Kirkuk, Kurdish soldiers known as pesh merga filed in and out of a command post near oil fields that emitted black smoke from gas flares. The Kurdish region exports about 550,000 barrels of oil a day, including oil from fields near Kirkuk, earning about $ 8 billion annually.
Kamal Karkokly, the Kurdish commander for the Dibis area, said he expected an attack by government forces at any time. “This is a very dangerous situation,” he said, sitting below a detailed military map of the region. “If they try to attack, the result will be bad for them and for us, and for the whole region.”
The mayor of Dibis, Abdullah al-Salihy, said that oil fields outside the town were protected by a brigade of pesh merga, but that he still fears an Iraqi military attack. If that happens, he said, he will join the pesh merga. “Everybody here carries their weapons home with them at night,” he said.
In downtown Kirkuk on Sunday, Sheikh Hatim al-Asi, spokesman for a regional militia that is part of government-sanctioned units known as popular mobilization forces, said Baghdad was determined to reclaim the areas. “We prefer to solve this by negotiations, but if we are forced to fight, we will,” Mr. Asi said.
In downtown Kirkuk, well-armed local and federal police officers were out in force on Sunday to “reassure the people that everything is under control,” said the commander of the Kirkuk police, Gen. Khatab Omar. The referendum alarmed many Arabs and Turkmens in Kirkuk who say they want to live under Iraq’s central government, not under Kurdish regional control.
“We need to resolve this,” he said of the dispute between the Kurds and Baghdad. “It’s frightening our citizens.”