Officials in the State and Defense departments told senators yesterday that they know relatively little about the enemy in Iraq but they believe thousands of hidden fighters are more organized than previously thought and are likely to continue deadly attacks in coming weeks and months.

Admitting that U.S. officials have underestimated the insurgency, Deputy Secretary of State Richard L. Armitage told the Senate Armed Services Committee that a series of attacks across Iraq in recent days indicate that the attackers have a "central nervous system" that is showing increased coordination and effectiveness. While the U.S. military expects heightened violence as Iraq approaches the transfer of limited power to an interim government next week, the sophistication of recent attacks has come as a bit of a surprise, according to testimony yesterday.

Armitage, Deputy Secretary of Defense Paul D. Wolfowitz and Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, told senators that they continue to believe that the insurgency is made up of a small minority of extremists and former members of Saddam Hussein's government who are bent on disrupting the drive for democracy in Iraq. But what was previously envisioned as a faltering insurgency has evolved into a significant security problem and a largely unknown quantity.

"I don't think anyone in this administration yet can tell you with a great deal of accuracy who they are and how many they are," Armitage said, responding to concerns from Sen. Pat Roberts (R-Kan.). Armitage said that attacks are probably the handiwork of former regime elements and those loyal to al Qaeda terrorist Abu Musab Zarqawi. "I said one of our mistakes was that we didn't understand there was a central nervous system. Well, clearly there is."

With major combat operations in Iraq over for more than a year, the level of violence in Iraq is far higher than U.S. officials predicted. Attacks on the coalition and Iraqi civilian targets are expected to continue through the official handover of limited power Wednesday, and officials said yesterday they are concerned that such attacks could increase as the country heads toward its first democratic elections in December or January.

Armitage said he expects those who carried out recent attacks to "reload and try again," predicting they will "really exercise themselves" in coming months.

The enemy is now using car bombs and more conventional warfare in targeting government officials and the new Iraqi police force, and senators heard yesterday that well-financed extremist leaders appear to be recruiting unemployed young men with the promise of a paycheck. Officials said yesterday that some of the ongoing attacks appear to be organized out of Fallujah, where officials believe extremists are hiding amid civilians.

Wolfowitz said he believes most of the insurgency is made up of people who did not surrender after the U.S. invasion on April 9, members of Hussein's regime who quietly melted away from the military and security forces. For example, he said, had U.S. forces concentrated earlier on the buildup of forces in Najaf, cleric Moqtada Sadr "might not have gotten out of control the way he did."

Wolfowitz said Wednesday's handover to an interim government should make a major difference to Iraqis because the United States would no longer be viewed as an occupying force. Then, he said, Iraqis can fight for their own freedoms and unify against those trying to block that effort. So far, more than 200,000 Iraqis have signed up, and U.S. officials expect the new Iraqi army, police force and border patrol to be ready by next year. Coalition forces -- including 141,000 U.S. troops -- will remain in Iraq indefinitely to secure the new government.

Sen. John McCain (R-Ariz.) grilled the panel on the progress in Iraq, complaining that the administration acknowledges it has not accomplished its goals in Iraq but is reluctant to admit to mistakes.

"It's interesting. Things didn't turn out as we anticipated they would, yet we didn't do anything wrong," McCain said.

Also yesterday, Sen. John W. Warner (R-Va.), chairman of the Armed Services Committee, said he wants to see an overdue Army investigative report into detainee abuse at the Abu Ghraib prison. The investigation, which focuses on the role of military intelligence soldiers at the prison, has been delayed so that a higher-ranking general could be assigned to look at the actions of Lt. Gen. Ricardo S. Sanchez, who commands U.S. troops in Iraq. Wolfowitz said Lt. Gen. Anthony R. Jones is now in charge of the inquiry and will issue a report when he can.