Updated

U.S. intelligence agencies have identified several guerrilla groups, including one whose name calls for Saddam Hussein (search)'s return, that they now believe are behind much of the anti-U.S. violence in Iraq.

The top general in the region, Gen. John Abizaid (search), estimated Thursday that insurgent fighters in Iraq total no more than 5,000, and he said the largest and most dangerous groups are Saddam loyalists.

"I would say that this group of Baathists [Saddam loyalists], by far, represents the greatest threat to peace and stability," the general said.

Abizaid, the chief of U.S. Central Command, did not provide details, but said the insurgent forces have considerable training and supplies, plus money from stashes left over from Saddam's rule and from sources outside Iraq "that are not clear to us."

Other officials, who spoke to The Associated Press on condition of anonymity, described some of the major guerrilla groups they believe have taken shape in Iraq.

The most significant, The Return Party (search), is composed primarily of members of Saddam's Baath Party, the officials said. Another, Muhammad's Army, appears to be run by former chiefs of Saddam's security services.

Other threats include Sunni Islamic extremists, some from outside the country, and Shiite extremists who may be receiving support from others in Iran.

Much remains murky about these opposition forces, the officials acknowledged. And most of the major bombings and attacks against U.S. and other Western targets remain unsolved.

But defense and other American officials familiar with Iraqi intelligence say they are making some headway into characterizing the guerrillas. Identifying the groups and their support structures has been a major stumbling block to U.S. efforts in Iraq.

The groups are all using classic guerrilla tactics: bombings, snipings and hit-and-run attacks, and at least some are using homicide bombers, officials said. While the groups have varying degrees of organization and capability, their emergence in the months since Saddam was ousted suggests the U.S. could be facing a sustained insurgency from several fronts.

Chief among the identified guerrilla groups are:

-- The Return Party: Considered the most significant insurgent group in Iraq, although officials could not put a figure to its size. It is composed primarily of members of Saddam's Baath Party and maintains the party's prewar structure, with regional and local organizations.

Its name refers to the return of Saddam to power, which is the group's goal. The group is strongest in Baghdad as well as in central and western Iraq, in the provinces of Salahaddin and Anbar. It is also thought to be distributing materials aimed at frightening Iraqis away from cooperating with American forces.

One U.S. defense official said members of this group may be working with Islamic extremists.

-- Muhammad's Army: This group, also seeking to return Saddam to power, consists of at least several hundred former members of Iraq's intelligence and security services.

Members are thought to be primarily targeting Iraqis who are working with American and other occupation forces. A group with this name is one of several that claimed responsibility for the Aug. 19 bombing of U.N. headquarters in Baghdad.

The group is strongest in Baghdad, Mosul and Fallujah. How closely it is working with the Return Party is unclear.

-- Saddam's Fedayeen: Elements of the Fedayeen, one of Saddam's prewar irregular militias, continue to operate separately from other insurgents, U.S. officials say.

Most of the Iraqi forces supporting Saddam's return are from the country's Sunni minority but are thought to be motivated by politics rather than religion.

U.S. officials also have identified several groups they label as extremist:

-- Muntada al-Wilaya: A Shiite extremist group operating in Baghdad and southern Iraq. It wants to eject American forces and set up an Islamic state like Iran's. American officials have suspicions that this group is linked to the Qods Force, an Iranian special forces unit that reports to the religious government in Tehran, and to Lebanese Hezbollah.

-- Ansar al-Islam: A Sunni group, composed primarily of ethnic Kurds from Iraq's north, that U.S. officials say has ties to the Al Qaeda terror network. Members may be acting as local fixers for foreign Al Qaeda members entering Iraq.

-- Abu Musab Zarqawi: A man, not a group, with Al Qaeda ties. This Jordanian is thought to be working with Ansar al-Islam but also leads his own network in Iraq.

Various U.S. officials have also spoken generally of "foreign fighters" as a threat. While some may be operating under Al Qaeda's umbrella, many are thought to be lone actors or small groups inspired to wage jihad, or religiously motivated war, against U.S. forces.

In addition, the U.S. defense official said members of more than two dozen non-governmental organizations operating in Iraq are suspected of supplying logistical and intelligence assistance to anti-U.S. fighters. The official did not name any, however.