He also said "hundreds" of Al-Qaeda in Iraq had been captured or killed in August.
Rear Admiral Mark Fox said Abu Mohammed al-Afri, also known as Abu Jassam was killed in an air strike on September 3, southwest of the northern city of Mosul.
Mr Fox said Abu Jassam was an associate of Abu Ayyub al-Masri, the Egyptian-born leader of Al-Qaeda in Iraq, the local affiliate of Osama bin Laden's global jihadist group.
Entire families were wiped out after suicide bombers blew up four lorries packed with explosives in two villages inhabited by the ancient Yazidi religious sect in the northern province of Nineveh.
The bombers struck the villages of Qataniyah and Adnaniyah, razing them completely, in the deadliest attack anywhere in the world since the September 11, 2001 strikes in the United States.
Yazidis, who are estimated to number several hundred thousand worldwide, speak a dialect of Kurdish but follow a pre-Islamic religion and have their own cultural traditions.
They believe in God the creator and respect the Biblical and Koranic prophets, especially Abraham, but their main focus of worship is Malak Taus, the chief of the archangels, often represented by a peacock.
Followers of other religions know this angel as Lucifer or Satan, leading to popular prejudice that the secretive Yazidis are devil-worshippers.
The community has attempted to remain aloof from the vicious sectarian and political conflicts gripping much of the rest of Iraq, but in recent months relations with nearby Sunni Muslim communities have worsened dramatically.
Mr Fox said the US military "surge" was applying pressure on Al-Qaeda in Iraq and in August troops had captured or killed hundreds of its militants.
"The combination of the surge, coupled with the increased capacities of the security force, the partnership with the Iraqi people, local tribes and sheikhs have significantly degraded Al-Qaeda in Iraq's control network, car bomb networks and their ability to produce propaganda through media," Mr Fox said.
The US military's surge strategy was launched on February 14 to rein in the bloodshed in Iraq and allow space for politicians to step up the national reconciliation process.
The top US officials in Baghdad, General David Petraeus, head of US-led forces in Iraq, and Ambassador Ryan Crocker, are to testify in Washington from today whether the strategy has worked or not in achieving its goal.