MOSUL, Iraq — A new Iraqi military offensive is under way in this still violent northern city, but the worry is not only the insurgents who remain strong here. American commanders are increasingly concerned that Mosul could degenerate into a larger battleground over the fragile Iraqi state itself.

The problems are old but risk spilling out violently here and now. The central government in Baghdad has sent troops to quell the insurgency here, while also aiming at what it sees as a central obstacle to both nationhood and its own power: the semiautonomous Kurdish region in the north and the Kurds’ larger ambitions to expand areas under their control.

The Shiite-led government of Prime Minister Nuri Kamal al-Maliki is squeezing out Kurdish units of the Iraqi Army from Mosul, sending the national police and army from Baghdad and trying to forge alliances with Sunni Arab hard-liners in the province, who have deep-seated feuds with the Kurdistan Regional Government led by Massoud Barzani.

The Kurds are resisting, underscoring yet again the depth of ethnic and sectarian divisions here and the difficulty of creating a united Iraq even when overall violence is down. Tension has risen to the point that last week American commanders held a series of emergency meetings with the Iraqi government and Kurdish officials, seeking to head off violence essentially between factions of the Iraqi government.

“It’s the perfect storm against the old festering background,” warned Brig. Gen. Raymond A. Thomas III, who oversees Nineveh and Kirkuk Provinces and the Kurdish region.

Worry is so high that the American military has already settled on a policy that may set a precedent, as the United States slowly withdraws to allow Iraqis to settle their own problems. If the Kurds and Iraqi government forces fight, the American military will “step aside,” General Thomas said, rather than “have United States servicemen get killed trying to play peacemaker.”

The competing agendas of the Kurds and central government have nearly provoked violence before, but each side eventually grasped the risks. That may be the case now. At the moment, the Americans are hoping to refocus each side on fighting the insurgency rather than each other.

But the tensions underline that achieving basic security is only the first step toward deeper progress in Iraq — and that much remains, bitterly, unresolved.

Mosul falls outside the borders of the Kurdish region, but Mr. Barzani’s Kurdistan Democratic Party came to control the provincial government after Sunni Arabs boycotted the provincial elections in 2005. The Kurds say, however, that they will not abandon the city until they reclaim five areas in Nineveh Province, putting them on a political collision course with the central government.

Tense personal relations between Mr. Maliki and Mr. Barzani worsened, officials on all sides say, after a standoff in September between the Iraqi Army and the Kurdish security forces, the pesh merga, in eastern Diyala Province. American forces helped contain that confrontation.

More broadly, the two men do not see eye to eye on issues as fundamental as the sharing of oil resources, the resolution of disputed internal borders and the shape of the Iraqi nation. The Kurds want a loose federation, while Mr. Maliki, playing on nationalist sentiments, is increasingly pushing for a strong central government.

Relations have deteriorated to the point that the Kurdish leadership has described Mr. Maliki as a new Saddam Hussein, recalling how Mr. Hussein ruthlessly crushed the Kurds in the 1980s. The borders of Iraqi Kurdistan were established as an internationally enforced security zone in 1991.

Testing Loyalties

In this latest offensive against insurgents, Mr. Maliki has been pushing to lessen Kurdish military influence here, and that is testing loyalties at a delicate time.

Mr. Maliki sent nearly 3,000 national policemen from Baghdad to Mosul to prop up the local force. The officers, almost all Shiites and Sunni Arabs, will be in charge of the overwhelmingly Sunni Arab west side of the city.

Predominantly Kurdish units of the army stationed in Nineveh are slowly being replaced by the mainly Sunni Arab and Shiite contingents.

The Defense Ministry also recently appointed Maj. Gen. Abdullah Abdul-Karim, Mr. Maliki’s brother-in-law, as the new commander of the Second Division on Mosul’s east side. Mr. Barzani, sensing a plot to purge the Iraqi Army in the north of its Kurdish leadership, personally intervened recently to freeze a ministerial order to transfer 34 Kurdish officers, said Col. Hajji Abdullah, a battalion commander in the Second Division.

“If the Arabs do not change now, things will get worse and I see confrontation,” Colonel Abdullah said.

In the turmoil, he and another officer in the division, Brig. Gen. Nadheer Issam, say their loyalties are first and foremost to Kurdistan.

“If I were made to choose, I would not even think for a second — I would leave the army,” General Issam said. “We have sacrificed too much fighting the Baathists,” he added, referring to Mr. Hussein’s political party.

The United States has relied on Kurds from the very beginning in Mosul. Ignoring longtime enmities between the city and Mr. Barzani’s party, American Special Forces units accompanied pesh merga fighters beholden to the party when they took Mosul in April 2003. The United States drafted more pesh merga units into the city in 2004 and 2005 when the whole provincial government and the police force collapsed at the hands of insurgents.

Although many of the pesh merga units in Nineveh were merged into the national army, an estimated 5,000 men remained from an elite Kurdistan corps in the province’s north. All these actions have stoked anger in Mosul toward Americans and Kurds.

Karam Qusay, who works in the Zuhoor neighborhood of Mosul, said he wanted the city to be free of the Kurdish military presence, both in the army and outside of it.

“We wish they would leave,” he said. “We despise them.”

Mosul’s allegiance to Mr. Hussein was so staunch that the city was known as the “regime’s pillow.” Now Mr. Maliki appears to be trying to win over the city by playing on grievances toward the Kurds.

“The government wants to extend its authority, and this clashes with the will and ambitions of the Kurds,” said Maj. Ali Naji, a Sunni Arab in one of the army units sent recently from Baghdad. “I predict fighting between Iraqi forces and the pesh merga.”

Sami al-Askari, one of Mr. Maliki’s senior advisers, said he hoped that talks between his boss and Mr. Barzani would head off any such confrontation.

But he made the government’s position clear: that the presence of Kurdish forces outside of the national army and beyond the borders of Kurdistan was “unlawful.” And he said the refusal of Kurdish officers in the Iraqi Army to obey their transfer orders from Nineveh was a “mutiny that must be severely punished.”

The repercussions of a face-off between Baghdad and the Kurds in Mosul, Iraq’s second largest city, would be far more serious than the recent tensions in eastern Diyala.

Tenuous Security

Nineveh, wedged between Iraqi Kurdistan and Syria and close to Turkey, remains a focal point for a number of Sunni insurgent groups linked to Al Qaeda in Mesopotamia, the homegrown terrorist group that American officials say is led by foreigners, and to the Baath Party. Both are fighting the Americans, Mr. Maliki’s government and the Kurds.

Despite numerous offensives by American and Iraqi forces since the start of the year, security remains tenuous at best. This was underscored this month when 2,270 Christian families, according to the Human Rights Ministry, fled Mosul after a number of killings and other attacks against Christians.

The overall level of violence has dropped in Mosul to 9 or 10 attacks a day from an average of 40 a day a year ago.

Yet killings continue, and fear is palpable. Judges are so intimidated or corrupt that the Iraqi government has flown in judges from Baghdad. Their main job is to issue arrest warrants for wanted suspects.

People other than Christians are also being attacked. A senior provincial official was killed as he left a mosque last month. Even a man who makes tea in the provincial building was recently killed in what is probably the most secure part of the city, said an American official working with local authorities.

In his push to subdue Mosul and marginalize the Kurds, Mr. Maliki is trying to curry favor with disaffected Sunnis. Last week he sent his deputy, Rafie al-Issawi, a Sunni, here with promises of a reconstruction and investment initiative that would be coordinated this time by respected Sunnis from Mosul.

More significant, Mr. Maliki is courting former army officers and tribal leaders like Sheik Abdullah al-Humaidi, who leads the powerful Shammar tribe in western Nineveh. All are strong nationalists who believe that Kurds must be confined to the borders of Kurdistan drawn after the Persian Gulf war in 1991.