FORWARD OPERATING BASE CALDWELL, Iraq (CNN) -- Although America's top general in Iraq called al Qaeda "the wolf closest to the sled," he said sectarian fighting among militias fueled by Iran could be the biggest long-term challenge for Iraq.
Gen. David Petraeus, in an interview with CNN's Jim Clancy near the Iranian border in Diyala province, said, "Militias could potentially be the long-term problem for Iraq, if you assume that we can continue to make progress against al Qaeda," Petraeus said.
He said he is in a "show-me mode," waiting to see if Iran honors a pledge to stop the flow of arms, money and training from Iran into Iraq that has helped both Shiite and Sunni militants.
"Al Qaeda remains the wolf closest to the sled, if you will. The enemy that is always bent on reigniting sectarian violence, causing the most horrific casualties, damaging the infrastructure in the most difficult way. So you cannot lose focus on al Qaeda."
But, Petraeus added, there was "no question" that Iranian arms were ending up in the hands of the Iraqi militias and there was "no debate" that six Iranians detained by the U.S. military in northern Iraq are Iranian Quds force members, the Iranian unit accused by the United States of training and arming insurgents.
"There's no question, absolutely no question that Iran is providing advanced RPGs [rocket-propelled grenades], RPG 29s," Petraeus said.
"It has provided some shoulder-fired, Stinger-like air-defense missiles. It has provided the explosively formed projectiles and it has provided 244 mm rockets, in addition to mortars, mortar rounds and other small-arms ammunition."
Petraeusalso said the Iranians "are implicated in the assassination of some governors in the southern provinces."
He said one indication the Quds force controls Iranian policy is that Iran's ambassador to Iraq, Hassan Kazzem Qomi, is a member of the the group. Qomi, who has diplomatic immunity, could not immediately be reached for a reaction.
Gen. Petraeus said the Iranian ambassador has given his Iraqi counterpart assurances Iran would stop the supplying and training of insurgents.
"They had two sessions," he said. "Numerous Iraqi leaders have gone to Tehran and asked that they stop very, very directly, stop the legal assistance. There have been sub-ambassadorial meetings as well. And there have been assurances in return actually from Iran to Iraqi leaders and we are waiting to see if those assurances bear fruit or not frankly. We are very much in the show-me mode right now. We would love to see that."
Petraeus reiterated that Iranians detained by the United State recently in northern Iraqare Quds Force members. One of them was arrested recently in Sulaimaniya and five others were arrested in Irbil.
The U.S. military has intercepted caches of explosively formed projectiles -- a more sophisticated and powerful type of roadside bomb -- and other weapons from Iran in recent months, but Petraeus said stopping the regular flow of arms to the militias is a challenge.