Facing an entrenched insurgency in Iraq whose ranks have grown significantly over the past year, the Pentagon has devised a new military strategy aimed at driving a wedge between various factions, defense officials said.

The strategy stems from what the officials said is a deeper understanding of an insurgency that has gained strength in recent months and proved tougher and more resilient than expected. Once viewed as little more than a few thousand embittered remnants of Saddam Hussein's government, the hard-core militants in Iraq are now estimated by senior U.S. military officers to number as many as 12,000.

The dominant element of the insurgency, the officials said, is a loose group referred to in U.S. military documents as "Sunni Arab rejectionists," consisting largely of former members of Hussein's government. These are onetime military officers and intelligence agents who U.S. officials have come increasingly to believe had some kind of plan to reorganize into cells and wage an insurgency if U.S. forces invaded.

Filling out the resistance, the officials said, are an assortment of Islamic extremists, some homegrown, such as the militia led by radical Shiite cleric Moqtada Sadr, and some foreign, such as those associated with Jordanian-born Abu Musab Zarqawi, plus a mix of criminals, financiers and other "facilitators" operating inside and outside Iraq and having access to substantial sums of money.

The new Pentagon plan, devised over the summer, centers on enticing more Sunnis into the political process while targeting the Islamic extremist groups for elimination. It depends heavily on building up Iraqi security forces more successfully than in the past year and breaking the bureaucratic logjams that have stymied flows of reconstruction aid into formerly rebel-held cities such as Samarra to win over civilian populations.

"The aim is to drive a wedge between the Sunni Arab rejectionists and the incorrigibles," said one senior official involved in policymaking on Iraq. "Many in the rejectionist group feel disenfranchised and are being intimidated. They need to be relieved of that yoke and engaged, while the extremists need to be isolated, captured or killed."

U.S. forces face substantial obstacles in bringing their plan to fruition. Commanders have identified 22 cities and towns in Iraq that must be brought under the control of the Iraqi government before nationwide elections, scheduled for January, can be held. The status of those cities is being assessed periodically by U.S. military commanders, based on a matrix that rates the insurgent threat in the area, the readiness of local Iraqi security forces and the functioning of local government services.

Since the start of the holy month of Ramadan two weeks ago, insurgent attacks against Iraqis and U.S. and coalition forces have risen more than 25 percent, to about 80 a day. Pentagon figures show that about 80 percent of the attacks have been concentrated in four of Iraq's 18 provinces: Baghdad, Anbar, Salah ad Din and Ninawa, all areas heavily populated by Sunnis.

Moreover, the notion that the use of military force against some insurgent groups can be balanced with political and economic enticements aimed at others is a risky one, say experts on Iraq inside and outside the government. They warned in interviews that U.S. firefights and aircraft attacks have themselves fed the insurgency, turning the relatives of slain militants and civilians into new insurgents.

"We don't understand when someone kills a brother, it calls for revenge killing," said Barbara K. Bodine, a State Department official who served in Iraq last year and now is a fellow at the Institute of Politics of Harvard's Kennedy School of Government. "We underestimate our daily humiliation of Iraqis."

Within the U.S. military, senior officers involved in the Iraqi effort say one of the greatest difficulties will be breaking an intensified campaign of intimidation being waged by insurgents against Iraqis and others associated with the U.S.-led coalition or Iraq's interim government. Borrowing tactics used by Hussein to instill fear in any who would consider opposing him, insurgents have embarked on an increasing spree of assassinations, suicide bombings and kidnappings targeting Iraqi political figures, bureaucrats, police, interpreters, international aid workers and others portrayed as collaborators.

As examples of how the attacks have appeared aimed increasingly at undermining existing Iraqi authorities, in the past 10 days gunmen in Baquba assassinated the deputy governor of the Iraqi province of Diyala; a suicide car bomber in Mosul killed three Iraqi government employees, while another wounded an Iraqi general. Also in Mosul, the sheik who chaired an association of tribal chieftains in the north was assassinated. And in Irbil, gunmen killed the chief of police.

With Iraq at a critical stage, as nationwide elections approach and U.S. and Iraqi forces prepare for possible new offensives against insurgent strongholds, several senior military officers and civilian defense officials agreed recently to discuss the Pentagon's strategy and current view of the insurgency. Some of the strategy was first reported in the New York Times earlier this month.

They said no single unifying leader of the insurgency is thought to exist. While some ad hoc contacts appear to occur among factions, they said, there is little evidence of a national network of coordination or a single ideological vision. If a common current runs through many of the groups, they said, it is a strong nationalist opposition to a foreign presence.

Estimates of the size of the insurgency vary. A senior military officer who works the issue and who last autumn put the number of "committed fighters" at 5,000 -- a figure disclosed publicly in November by Gen. John P. Abizaid, the top U.S. commander in the Persian Gulf region -- said that he now estimates they have grown to as many as 12,000.

He said the insurgency has expanded in part because of mounting Iraqi irritation with the presence of U.S. troops in Iraq. Other contributing factors, he said, include the scandal over mistreatment of prisoners at U.S.-run detention centers and the rise of Sadr's militia.

The strategy to differentiate between extremist elements who are considered lost causes and those in the Sunni resistance who might be persuaded to drop their opposition was formalized in Pentagon planning documents in August and has been refined several times since, defense officials said. Alluding to the plan, Defense Secretary Donald H. Rumsfeld has spoken in recent weeks of methodical attempts by U.S. and Iraqi authorities to reach political accords with resistance groups. Iraq's interim prime minister, Ayad Allawi, who took office in June, also has made clear his desire to extend an olive branch to some Sunni militants.

In cases where these diplomatic efforts fail, Rumsfeld has signaled the likely use of military force, as was demonstrated four weeks ago in Samarra. Pentagon officials said the most critical test will come in an increasingly likely assault on more than 3,000 insurgents in Fallujah and Ramadi.

Over the past year, the U.S. view of the insurgency has evolved. Rumsfeld initially spoke dismissively of enemy fighters in Iraq as "dead-enders" -- referring to remnants of Hussein's Baath party, elements of his Special Republican Guard and other specialized units. As attacks continued, many were blamed on al Qaeda and affiliates such as Ansar al-Islam and other foreign fighters said to be entering from Syria.

More recently, senior officials have described an expanded and deeply entrenched array of Sunni cells in particular. They have cited evidence of plans by Hussein's closest advisers before the U.S. invasion to regroup in small cells afterward. They also say some former Baathist officials dispersed after the war began and began providing financing from outside as well as inside Iraq.

One senior defense official said more than a dozen "financial people" from Hussein's government have been identified funneling money from Syria to insurgents in Iraq. Izzat Ibrahim Douri, a former senior Baath Party official, is among those said to have traveled to Syria to help set up a support network. He is now believed to be back in Iraq and playing a significant role in coordinating attacks.

"The real enemy are the FRE," said the senior military officer, using the abbreviation for former regime elements. "But the problem with the FRE is they're so ingrained, so insidiously situated within Iraqi society."

Citing their ruthless violence and brutal intimidation, he likened them to the New York mafia in the 1930s, "where they owned blocks, where people wouldn't tell on them because they knew they'd have their throat slit." He called them "a very difficult target."

The view of Iraq's insurgency as a disparate assortment of groups is supported by a number of experts outside the U.S. government. While they describe the insurgency as lacking unity or a long-term vision, they also say it seems not to have suffered as a result.

"The insurgents may have calculated that their success does not now require an elaborate political and socioeconomic vision of a 'free' Iraq," said Ahmed S. Hashim, professor of strategic studies at the Naval War College and a consultant to Abizaid's Central Command. "Articulating the desire to be free of foreign occupation has sufficed to win popular support."

"The insurgents represent different philosophies but they all want to get us out of Iraq," said Bodine, the former State Department official. "There is no clear national leadership and no politics -- at least not yet -- but we're not far from a time when it could become national."

W. Patrick Lang, a former Army colonel and Middle East specialist in the Defense Intelligence Agency, said the insurgents were pursuing "a strategy of isolation of the occupier."

Iraqis arrive after two nearly simultaneous bomb attacks targeted Iraqi government buildings and a military convoy in Mosul. Three people were killed in last week's attack.