One of the officials noted that coalition forces thought they had delivered a major blow in January with the capture of Zarqawi's principal bomb maker in the capital. But since then, the official said, "car bombs are way up in Baghdad."
Overall, the officials said, the insurgency in Iraq is divided into three "clumps": religious extremists such as Zarqawi; former members of the Baath Party of Saddam Hussein; and disparate Iraqi groups acting out of local or national interests.
The officials described a steady flow of Saudis, Yemenis and other Arab nationals into -- and, in some cases, out of -- the country. But officials said foreign fighters accounted for less than 10% of the insurgents in Iraq.
Zarqawi's reported success in recruiting Iraqis to his cause comes as frustration is mounting among the minority Sunni Arabs, who fear they will be marginalized in the new Iraq and are prepared to fight its emergence.
The CIA and other agencies have resisted pressure to provide an estimate of the number of insurgents in Iraq, partly out of concern that it would foster the impression that there is a finite population that can be stamped out.
Rather, officials said intelligence analysts had noted that there were about 800,000 to 1 million Iraqi Sunni Arab men of military age who represent the pool of potential insurgents. How many might turn to violence depends on several factors, starting with the extent to which Sunnis are satisfied with their stake in any new government.
Some Sunnis have objected to the draft constitution that is to be presented to Iraqis in a national referendum next month. The community's sense of estrangement could be heightened if the document is passed, as is likely, over its objections.
"They're going to be extremely disappointed when they fail, and they're going to believe this is the result of fraud and being cheated out of what they deserve," one of the U.S. officials said. "There's going to be some real ratcheting up of Sunni disaffection with the process."
The trial of Hussein, to begin next month, is also likely to add to a sense of victimization among Sunnis, analysts say.
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Times staff writer Borzou Daragahi in Baghdad contributed to this report.
