An audio recording has been released on the web claiming to feature the voice of al-Qaeda's leader in Iraq, urging Sunnis to attack the country's Shias.
Zarqawi last appeared in a video recording earlier this year
The taped voice - allegedly that of Jordanian militant Abu Musab al-Zarqawi - says Shias have long collaborated with foreign invaders in Iraq.
It urges Sunnis to resist attempts at reconciliation with the Shias.
The CIA says voice analysis confirms the tape is genuine. Zarqawi has been blamed for scores of attacks in Iraq.
The fugitive militant last appeared in a video recording earlier this year, in which he was seen firing a weapon in the desert and reproaching the US for its "arrogance and insolence".
The US released a letter in early 2004, allegedly written by Zarqawi, that said fomenting strife between Iraq's Sunnis and Shias could galvanize the resistance to US forces.
Al-Qaeda has been blamed for scores of bombings targeting Shias and US forces in Iraq.
The speaker in the audio recording urges Sunnis to "wake up, pay attention and prepare to confront the poisons of the Shia snakes".
"Forget about those advocating the end of sectarianism and calling for national unity," the speaker says.
The recording also describes Iraq's highest-ranking Shia cleric, Ayatollah Ali al-Sistani, as an "atheist" and lambasts Shia militia groups for attacking Sunnis in their houses on the pretext of searching for insurgents.
The speaker in the tape also attacks targets beyond Iraq.
He describes Lebanon's Shia militia group, Hezbollah, as a "shield" protecting Israel from attack.
He also mocks the Iranian leader, Mahmoud Ahmadinejad, for "screaming and calling for wiping Israel from the map" while failing to back up his words with actions.