The United States is worried over indications that Muslim extremists from the Hizbollah, Islamic Jihad and Iranian Shiite groups have started entering Iraq.

The fear is that these radicals could incite more suicide bombings aimed at US troops, a media report says.

According to Time magazine, a major American security concern in Iraq, once President Saddam Hussein is ousted, is not the remnants of the government, but the presence of such radicals.

"It's the presence of radicals who may owe their allegiance to neighbouring regimes that have their own agenda," it said quoting a senior US official.

It quotes a US counter-terrorism official as saying intelligence reports aren't conclusive about the number of terrorists who have entered, but small groups of Islamic Jihad and Hizbollah agents have infiltrated the country, and "We've got people paying attention to both approaches, from Syria and Jordan." 

Yet, the magazine adds, it's far from certain that these groups could combine to form a significant threat. Hizbollah and the Islamic Jihad share few values with Saddam's Baathist nationalists.

The report adds that Iraqi Shi'ites and Iranian Shi'ites are not ideological soul mates and fears after Gulf War I that the two would join and carve out a separate state aligned with

Iran proved to be unfounded.

Some analysts, it adds, even speculate that the infiltration could aid US efforts to track down terrorists.