The speaker on the tape was identified as Abu Hamza al-Muhajir, also believed to use the name Abu Ayyub al-Masri.
He became the leader of al-Qaeda in Iraq after the killing of his predecessor Abu Musab al-Zarqawi in a US air strike in June.
“Place your hands in our hands … our enemy has unified his ranks, now is the time to unite,” said the speaker on the tape.
The speaker urged Islamists to “kill at least one American within a period not exceeding 15 days”.
Al-Muhajir was announced as al-Qaeda in Iraq’s new leader on June 12 and received Osama bin Laden’s endorsement in an audiotape from the al-Qaeda chief.
In the first major attack claimed by the group under al-Muhajir, it said it carried out the kidnapping and slaying of two American soldiers whose mutilated bodies were found in southern Baghdad on June 20.
Al-Qaeda in Iraq accounts for about 5 per cent of Iraq’s Sunni Arab uprising but its bombers have been responsible for some of the worst violence, often killing over 100 people in a single attack.