"By God, America will be defeated in Iraq," said the man identified as al-Zarqawi in what would be the first public appearance of one world's most-wanted militants in at least three years.
A US military spokesman said the video probably is authentic.
"At this time, we have no reason to doubt the authenticity of the tape and we are taking it at face value," an unnamed military spokesman in Baghdad said on Wednesday.
"This appears to be a direct reaction to the progress of forming a legitimate and permanent government for the people of Iraq, a process al-Qaeda in Iraq has always tried to undermine by every means possible," he said.
The spokesman was referring to the appointment of Jawad al-Maliki as the new Iraqi prime minister, a move that ended months of political deadlock.
In the past, al-Zarqawi has made statements only through audiotapes posted on the web.
Al-Zarqawi has been accused of
kidnappings and killings in Iraq
"When the enemy entered into Iraq, their aim was to control Iraq and the area," the man said in the video. "But here we have been fighting them for the last three years."
He lashed out at the Iraqi government for collaboration with what he called "Western crusaders".
"We believe that any government formed in Iraq ... by enemy agents who are falsely counted as Sunnis is a government of traitors," he said.
In the video, the man, sporting a beard, is shown meeting with fighters briefing him on events in the western Iraqi town of Ramadi.
"Your brethren in the information committee of the Mujahedeen's Consultative Council are happy to present the first video of the emir of the al-Qaeda Organisation in the Land of Two Rivers, Abu Musab al-Zarqawi," footage on the website said.
The man also announced the formation of an umbrella group for anti-US fighters.
He also was shown firing an automatic rifle. The message on the video was dated April 21.
Jordanian-born al-Zarqawi is blamed for deadly attacks in Iraq and the kidnapping and beheading of hostages and has a $25 million US bounty on his head.