The violent insurgency against U.S. occupation has slackened noticeably in the week since formal political authority was turned over to an Iraqi government. But according to Iraqi and U.S. sources, the bloodshed is far from over.

The insurgency is likely to persist as long as U.S. soldiers remain visible in Iraq, they said, because it cuts across several irreducible currents in the country's long-dominant Sunni Muslim minority. These include nationalism and Arab pride, local and transnational Islamic fundamentalism, and tribal loyalties cultivated by ousted president Saddam Hussein.

"If you don't remove the causes, you will never get rid of the resistance," said Saleh Mutlak, a onetime Hussein official who helped negotiate the ragged truce protecting Fallujah, the embattled city 35 miles west of Baghdad that is his family home and the insurgency's main stronghold.

One major expression of the insurgency, the Shiite Muslim uprising led by Moqtada Sadr, has been put on hold while Shiite leaders seek to enlist the rebellious cleric in the political process and persuade him to lay down his arms. For the time being, that truce has removed thousands of Iraqi youths from the conflict and halted what risked becoming a marriage of wartime convenience between Iraq's 60 percent Shiite majority and the 20 percent Sunni minority that traditionally has run the country.

But Sheik Abdul-Satar Abdul-Jabbar of the Council of Islamic Scholars, Iraq's preeminent Sunni organization, said the main, Sunni-based insurgency is likely to rage as long as Iraqis' sense of nationalism is bruised by the presence of U.S. military forces on their soil. The emergence of Prime Minister Ayad Allawi's interim government has changed that equation little, he said, because the insurgents regard it as an extension of the military occupation with no roots of its own in Iraq.

Abdul-Jabbar cited in particular a statement made in Washington on Friday by Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, predicting that 145,000 U.S. troops would probably be in Iraq for the next five years. "That will incite even more Iraqis to enlist in the resistance," Abdul-Jabbar said.

The increasingly frequent appearances of U.S.-trained Iraqi National Guard soldiers and policemen in the streets of Baghdad and other cities have drawn a favorable response from many Iraqis who express dislike for U.S. occupation troops but say they have wearied of the constant violence. In addition, many Iraqis, including prominent Sunni and Shiite leaders, shuddered at the number of Iraqi policemen and civilians killed by car bombs during a high point of violence on June 24.

Taken together, these developments could suggest public doubts about the insurgency, at least in its bloodiest forms. But Iraqi sources with knowledge of the insurgency pointed to what they said was a worrying trend in the opposite direction: a radicalization of young Iraqis caught up in the battle.

One Iraqi source estimated that at least 28 different insurgent groups have formed in recent months, some -- but not all -- based in Fallujah. This intensifies the danger of bloodshed, he and other Iraqi sources said, because the most youthful and rash of these groups are increasingly embracing tactics imported by foreign fighters, such as the car bombings of civilian targets.

"We are not under any illusion that we can eliminate international terrorism and opposition to the government. There still is more sting in that snake," said Minister of State Adnan Janabai.

U.S. officials have often called attention to their campaign to hunt down Abu Musab Zarqawi, an elusive Jordanian linked to Osama bin Laden's al Qaeda network who is the most prominent foreigner believed to be fighting here. Four airstrikes in recent days targeted what the U.S. officials said were safe houses in Fallujah used by his followers. But the same officials have also attributed to Iraqis nearly all the attacks against American soldiers or Iraqi security forces.

"We must admit that most of the violence in this country is carried out by people who were born and raised in this country," said a senior U.S. military official who spoke on condition of anonymity.

Zarqawi, whose soft, round face adorns wanted posters across the country, has been blamed for the car bombings, which are often carried out by suicide drivers. But Mutlak and Abdul-Jabbar said that in Fallujah, where most of the foreign fighters are reported to be concentrated, only a handful of those battling U.S. Marines entrenched outside the city are non-Iraqis.

The main components of Fallujah's fighting force, Mutlak and other Iraqi sources said, are the city's own fundamentalist Sunni Muslims, together with officers, soldiers and intelligence agents from the Hussein years, many of whom have family in Fallujah. Added to them, he said, are young men seeking revenge for family members killed by U.S. forces and other residents drawn into the battle out of solidarity with their neighbors and resentment over U.S. attacks on their home town.

The fighters and their leaders have more or less run Fallujah since the truce was arranged in early May. As a result, U.S. military officials have expressed worry that Fallujah may become -- or perhaps already has become -- an incubator for insurgency recruits.

The Fallujah Brigade, a security force under the command of former Iraqi army officers, was supposed to take control of Fallujah in early May in collaboration with the U.S. Marines. Instead, it has ceded authority to a sort of commune that has sprung up in recent weeks, guided by the Mujaheddin Advisory Council under the leadership of the town's senior Sunni cleric, Sheik Abdullah Janabi, and his main lieutenant, Sheik Dhafer Obeidi.

L. Paul Bremer, who was the U.S. administrator in Iraq until the transfer of political authority on June 28, arranged as one of his final acts to have warrants issued for the arrest of Janabi and Obeidi, according to Mutlak and Baghdad news reports. If U.S. troops tried to take the two Sunni clerics into custody, Mutlak predicted, Fallujah would erupt into even greater violence.

A similar order against Sadr in April touched off the weeks-long confrontation between the cleric's Mahdi Army militia and U.S. troops in the Najaf area, about 90 miles south of the capital. Mutlak said that episode showed that U.S. authorities are mistaken in confronting insurgent fighters rather than seeking to negotiate with them.

Allawi, the interim prime minister, while speaking out sharply against attackers, has also indicated a conciliatory attitude. His spokesman, Georges Sada, told reporters the government was working out an amnesty for resistance fighters who had not been involved in attacks that qualified as terrorism. In addition, Allawi has spoken of bringing former Baath Party and Iraqi army officials into the new system, in effect abandoning the de-Baathification program put in place by Bremer.

Many former intelligence agents and military officers have formed cooperative relationships with Islamic insurgents in Fallujah and elsewhere, even though Hussein's Baath Party was, in principle, secular. Their expertise in such areas as explosives and arms has boosted the firepower of insurgent organizations.

U.S. officials have speculated that former government officials might also still have access to money to help finance attacks. The most senior official from Hussein's regime still at large is Izzat Ibrahim, who specialized in internal security.

After Fallujah, the other main insurgency stronghold is Baqubah, a farming town 35 miles northeast of Baghdad. Residents there, as in much of the swath of central Iraq known as the Sunni Triangle, had strong ties to Hussein's Baath Party.

Successive U.S. Army units stationed in Baqubah have had particular trouble in Buhris, a village on the edge of the city where tribal sheiks were traditionally courted and paid off by Hussein's government. The web of tribal loyalties woven by Hussein's intelligence services around Baqubah has been difficult to pierce for U.S. military officers, whose men for months were shot at every time they entered Buhris.

Baqubah residents have said gunmen from elsewhere have also played a role in the violence there. Fighters proclaiming loyalty to Zarqawi were seen in the streets of Baqubah during the last eruption of large-scale fighting there on June 24.

"It's from outside," said Ali Abdul Kareem Madani, the senior Shiite Muslim cleric in Baqubah.

Baghdad itself, often a target of the bombings, was also discovered recently to be the site of several facilities where car bombs were being assembled and rigged into vehicles. About 50 people were arrested and bomb-making equipment was seized in a series of raids reported Saturday by the U.S. Army's 1st Cavalry Division.

One of the deadliest recent attacks, a series of car bombings in Mosul on June 24, was attributed to Ansar al-Islam, a small group in northern Iraq formerly associated with Zarqawi. The explosions, targeting Iraqi police buildings, killed more than 80 people in the city, 220 miles north of Baghdad.

Abdul-Jabbar said Ansar al-Islam was one of several names used by Zarqawi and his foreign or Iraqi followers. Others cited on Islamic Web sites include the Monotheism and Jihad Group, the Ansar al-Sunna Army and the Sharp Sword Against the Enemies of God and His Prophet, all of which have asserted responsibility for taking foreign hostages and in some cases beheading them.