Hezbollah, the Iranian-backed Shiite group, has established a significant presence in Iraq, but is not taking part in attacks on US forces inside the country, according to current and former US officials and Arabs familiar with the organization.
Iran is believed to be restraining Hezbollah from attacking US troops, and that is prompting a debate within the Bush administration about Iran's objectives, administration officials said.
Hezbollah's presence has become a source of concern as it is recognized by counterterrorist experts to have some of the most dangerous operatives in the world.
Both US and Israeli intelligence have found evidence that Hezbollah operatives have established themselves in Iraq, according to current and former US officials.
Separately, Arabs in Lebanon and elsewhere who are familiar with the organization say Hezbollah has sent what they describe as a security team of up to 90 members to Iraq.
The organization has steered clear of attacks on Americans, the US officials and Arabs familiar with Hezbollah agree.
US intelligence officials said Hezbollah operatives were believed to have arrived in Iraq soon after the end of major combat operations last spring, and had refrained from attacks on Americans ever since.
The CIA had not seen a major influx of Hezbollah operatives since that time, officials added.
"Hezbollah has moved to establish a presence inside Iraq, but it isn't clear from the intelligence reports what their intent is," one administration official said.
Based in Lebanon, Hezbollah is a Shiite Islamic group that is under Tehran's control. Syria, which dominates Lebanon and controls Hezbollah's supply lines from Iran, also plays a powerful role with the group.
Inside Lebanon, Hezbollah has taken on an increasingly political role, but it continues to pose a global threat.
The US has issued a US$25 million reward for the capture of Imad Mugniyah, the longtime chief of foreign terrorist operations; he is believed to have been behind a series of attacks against Americans in the 1980s, including hostage-taking operations in Lebanon.
More recently, Hezbollah has focused its activities on Israel, and is not believed to have launched a large attack against US interests since 1996, when, according to US government charges, it carried out the bombing of the Khobar Towers barracks in Saudi Arabia that killed 19 Americans.
In recent months, US troops have faced a deadly guerrilla campaign waged largely by the remnants of former Iraqi president Saddam Hussein's Baath Party government in the Sunni-dominated region of central Iraq.
Some foreign Arab fighters are believed to have infiltrated Iraq, but their role in attacks on US troops now appears to be less significant than US military and intelligence officials originally believed.
US forces have faced far less violence in Shiite-dominated southern Iraq than they have in the Sunni heartland.
The Shiites, though the majority of Iraq's population, suffered severe oppression under the Sunni-dominated government of Saddam, and have so far appeared more willing to accept the US military occupation.
But Iran's role in Iraq's Shiite community has been a wild card for the Bush administration.
Shiite-dominated Iran has a strong interest in influencing the political and religious direction of the country, particularly because some of the Shiite world's holiest sites are in the Iraqi cities of Karbala and Najaf.