It was the second battlefield controversy involving Rivera over the past 16 months.
Rivera, traveling with the 101st Airborne Division in Iraq, revealed tactical information and at one point told about an attack two hours before it took place, according to sources at the U.S. Central Command who asked not to be identified.
Fox's rivals, CNN and MSNBC, both reported Monday that Rivera had been kicked out of the country.
"During a live broadcast, Geraldo drew a map in the sand of where that unit was going. Not exactly what you're supposed to do out there," CNN anchorman Leon Harris said.
Shortly thereafter, Rivera delivered a report via satellite phone saying he was 60 miles from Baghdad. In the report, which first aired at 11:30 a.m. EST, Rivera labeled reports of his ouster "a pack of lies" spread by his former colleagues at NBC.
"It seems to me like some rats at my former network are spreading lies about me," he said. "They can't compete fair and square on the battlefield, so they're trying to stab me in the back. It's not the first time."
Rivera said he intended to ride into Baghdad in search of Saddam Hussein, "the Iraqi Hitler."
Pentagon spokesman Bryan Whitman said that while Rivera was not embedded with any military unit, any violations of embedding rules were taken very seriously.
"I have been in contact with the news organization and assured they are taking it seriously," Whitman said. "We will make an appropriate determination once we have evaluated all of the facts."
Fox News Channel executives did not immediately return telephone calls seeking comment.
This wasn't Rivera's first battlefield brouhaha. Sixteen months ago, while covering the war in Afghanistan, he generated criticism for being hundreds of miles from the site of a friendly fire incident he reported on.
Rivera reported in a Dec. 6, 2001, piece that he became emotional and choked up while standing on the "hallowed ground" in Afghanistan where "friendly fire took so many of our, our men and the mujahedeen yesterday." Rivera said he had recited the Lord's Prayer.
Rivera later admitted that he was several hundred miles from the site near Kandahar where three Americans were killed by an errant U.S. bomb.
In an interview, Rivera said he had confused the Kandahar deaths with another "friendly fire" incident that cost several Afghan lives in Tora Bora. But according to the Baltimore Sun, Pentagon information showed the Tora Bora incident occurred at least three days after Rivera's Dec. 6 report.
Rivera quit his talk show on CNBC to become a war correspondent for the Fox News Channel in November 2001. He said at the time he couldn't bear to stay on the sidelines during a big story.