The concerns were raised on Thursday after recent strikes against three high-profile Americans.
General John Abizaid, who commands US troops in the Gulf, came under attack in Falluja on Thursday when fighters shot rocket-propelled grenades at his convoy. Abizaid and his party were unharmed.
Defence officials said the Pentagon was aware that the Iraqi resistance might seek to infiltrate security forces created by the United States or to place a "mole" inside US-led civilian operations in the country.
"If you ask anyone, they'll all say, 'Yeah, it's a concern.' There's no two ways about it," said one official, speaking on condition of anonymity.
US officials targeted?
"Certainly we monitor the situation very closely," said another. "It's something that we've always been aware of and have taken steps to prevent."
But US officials said they had not concluded the attack against Abizaid resulted from inside information, saying the incident occurred in a city that has been among the most hostile to American occupation forces.
"They missed Wolfowitz. They missed Bremer. They missed Abizaid now. I hate to say this, but it's only a question of time before a high-ranking and high-profile figure gets nailed by the insurgents"
Resistance fighters have attacked two other senior US officials, who were unhurt.
On 26 October, rockets struck the Baghdad hotel where Deputy Defence Secretary Paul Wolfowitz was staying.
And on 6 December, a motorcade carrying US occupation administrator Paul Bremer was ambushed near Baghdad International Airport.
Defence analyst Charles Pena of the Cato Institute said the number of times fighters have been able to target high-profile Americans is disturbing.
'Question of time'
"That's something you cannot do that many times, I would argue, simply through sheer luck and coincidence. They have to be getting some kind of information," Pena said.
"They missed Wolfowitz. They missed Bremer. They missed Abizaid now," Pena said. "I hate to say this, but it's only a question of time before a high-ranking and high-profile figure gets nailed by the insurgents."
The US has recruited, trained and fielded 209,000 Iraqis into various security forces since last summer and many more Iraqis work with US civilian operations.
And there are just under 70,000 US-trained Iraqi police on the job around the country.
US officials admit there are challenges in weeding out Iraqis willing to give the resistance sensitive information about the location of key Americans or other subjects.
"There's been a lot of discussions about the Iraqi police being infiltrated. To tell you the truth, it's not that difficult to do"
Iraqi security forces
"There's been a lot of discussions about the Iraqi police being infiltrated. To tell you the truth, it's not that difficult to do," a defence official said.
"If you're going to do a background check on somebody, you're only going to know the obvious bad guys," the official added, noting that others who could pose a security threat could slip through the net.
Another official called the training and deployment of the Iraqi security forces "very much a success story", noting that these Iraqis work closely with US forces and more than 200 already have given their lives on the job.
"Can you rule out the possibility [of infiltration]? Of course not. But the vetting process itself is very thorough and has been conducted in a manner that has been effective."