Masri is believed to have formed al-Qaeda's first cell in Baghdad
The United States military in Iraq says a man detained in the northern city of Mosul is not in fact the leader of al-Qaeda in Iraq.
An earlier statement from the Iraqi defence ministry said that al-Masri had been captured.
But an American military spokeswoman, Peggy Kageleiry, said Abu Ayyub al-Masri had not been arrested.
She said confusion had arisen because a man with a similar name to the al-Qaeda in Iraq leader had been detained.
Al-Qaeda in Iraq has been blamed for or has claimed responsibility for some of the bloodiest insurgent attacks in Iraq since the US-led invasion in 2003.
Members of al-Qaeda are believed to have regrouped in the northern city since the beginning of the US troops "surge" last year.
Al-Masri is believed to have helped Abu Musab al-Zarqawi form the first al-Qaeda cell in Baghdad. Zarqawi was killed in June 2006.
In April 2007, he was named "minister of war" in the 10-man cabinet of the Islamic State of Iraq, an umbrella organisation of Sunni militant groups.