"The blood has been spilled in Iraq of more than 4,000 foreigners who came to fight," said the man, who identified himself as Abu Hamza al-Muhajir - also known as Abu Ayyub al-Masri - the leader of al-Qaida in Iraq, though the voice could not be independently identified.
The Arabic word he used indicated he was speaking about foreigners who joined the insurgency in Iraq, not coalition troops.
He also told Muslims on the recording that their holy month should be turned into what he calls a "month of holy war."
Ramadan began last weekend across the Muslim world.
Al-Masri appealed directly to insurgents in Iraq, urging them to take Westerners prisoner.
"I appeal to every holy warrior in the land of Iraq to exert all efforts in this holy month so that God may enable us to capture some of the Western dogs to swap them with our sheik and get him out of his dark prison," the voice said.
He was referring to the blind Egyptian sheik Omar Abdel-Rahman, imprisoned in the United States since 1995 for conspiring to blow up New York City landmarks.
Al-Masri is believed to have succeeded Abu Musab al-Zarqawi, who died in a U.S. airstrike north of Baghdad in June.
CBS News translator Khaled Wassef, whose job entails the constant monitoring of a plethora of Web sites where militants frequently post text, audio and video detailing their global operations, said the claim of 4,000 dead fighters is more important in symbolism than in quantity.
Wassef explained that al Qaeda in Iraq was likely demonstrating their ability to draw small armies of fighters from around the Muslim world into Iraq to wage Jihad against the coalition forces, and furthermore, the apparent ease with which those fighters can enter the country.