The tape surfaced Friday on an Internet site known as a clearinghouse for militant Muslim comment. Its authenticity could not be confirmed.
"As for you heroes of Islam in Fallujah, praise for your Jihad, praise for your nation, praise for your religion. (Have) one hour's patience, and then you will see the results," the speaker said after identifying himself as al-Zarqawi.
"Rejoice my nation. There is no doubt that God's victory is on the horizon," the speaker said, adding a challenge to "the Americans to show the truth of what goes on on the battleground."
The speaker also said Kurds and Shiites serving with the Iraqi forces have "sold their religion" and claimed the U.S.-Iraqi offensive in Fallujah had been blessed by "the infidel's imam," Grand Ayatollah Ali al-Sistani.
Al-Sistani, the leading Shiite cleric in Iraq, has issued no public statement on the Fallujah fighting. His silence is resented by radical Sunnis, who expected him to condemn the assault on the city.
The voice on the tape sounded like that on earlier tapes attributed to al-Zarqawi. The statement was peppered with verses from the Quran and poetry, typical of recordings linked to al-Zarqawi.
The statement appeared as American and Iraqi soldiers pushed deeper into the southern reaches of Fallujah, cornering militants being backed into smaller pockets of the city, on the fourth day of an offensive launched in part to clear out militants linked to the Jordanian-born al-Zarqawi.
His followers have beheaded at least six hostages: three Americans, a Briton, a Japanese and a South Korean. The United States has offered a $25 million reward for al-Zarqawi's capture or killing - the same amount as for Osama bin Laden.
The link to the five-minute tape was posted by an individual who signed himself Abu Maysarah al-Iraqi, known as the media coordinator of al-Zarqawi's group, al Qaeda of Jihad in Iraq.
In an unusual written introduction, al-Iraqi said the recording was "short and hasty" because of the "grave" circumstances. A longer statement would follow in which al Qaeda strategy would be outlined, al-Iraqi said.
Also Friday, another well-known Iraqi militant group, Ansar al-Sunnah Army, claimed in a statement on its Web site to have joined forces with al-Zarqawi's group and the Islamic Army in Iraq, which claimed responsibility for kidnapping two French journalists who remain missing.
Ansar al-Sunnah Army claimed to have carried out a "joint operation" with the two groups in Youssifiyah, 12 miles south of Baghdad. There was no way to confirm the veracity of the statement.
All three groups have claimed responsibility for several deadly attacks, kidnappings and beheadings of foreigners and Iraqis.
By Sarah El Deeb