Defense Secretary Robert Gates says Shiite extremists, not al-Qaida terrorists, are to blame for most of the recent U.S. military deaths in Iraq, and they're "clearly getting some fairly sophisticated and powerful weapons" from Iran.

Gates tells CNN's "State of the Union" that he's worried about the Iranian influence in Iraq and he thinks Prime Minister Nouri al-Maliki is beginning to take these Shiite groups seriously.

Gates says that the U.S. and Iraq are taking steps to try to limit the threat.

A Shiite militia group has claimed responsibility for an attack that killed five American troops on June 6. It was the single largest loss of life for American troops in two years.