Currently, man-portable air defense systems comprise five percent of attacks on U.S. aircraft in Iraq, Middle East Newsline reported. The army operates the AH-64D Apache, the UH-60 Black Hawk and the CH-47 Chinook on a 24-hour basis.

In an Oct. 19 briefing, Shanahan said Iranian SAM systems have forced army helicopter pilots to revise their tactics. He said the Iranian-origin missiles arrived in Iraq over the last three months.

"In the last several months, we have had an increased threat from systems that we had not seen in the first part of the year," Shanahan said.

Shanahan did not identify the Iranian missiles. Over the last 18 months, Iran was said to have received the SA-18 Igla-S man-portable missile from Russia.

Officials said U.S. Army helicopters have been equipped with additional sensors and diffusers in an effort to decrease the infrared signatures of the air platforms.

They said the biggest threat has been from shoulder-launched heat-seeking surface-to-air missiles, such as the SA-7.

"But if you ask the pilots, they would say that MANPADS is the biggest threat," Shanahan said.