Tucker Carlson, the sidelined prime-time Fox News host, on Tuesday released the first installment of what he said would be his new show on Twitter, potentially setting up a confrontation with the cable network, where he remains under contract until early 2025.

The 10-minute video, Mr. Carlson’s first extended commentary since Fox took him off the air in April, was similar to a stripped-down version of what his roughly three million Fox viewers would have seen on his nightly program.

There were no guests or produced segments — only a monologue from Mr. Carlson, in which he hit some familiar themes. He expressed sympathy for President Vladimir V. Putin of Russia and mocked President Volodymyr Zelensky of Ukraine. He accused the mainstream media of lying. He wrapped up by declaring that U.F.O.s and extraterrestrial life are “actually real.”

“As of today, we’ve come to Twitter,” Mr. Carlson said in the video. “We’re told there are no gatekeepers here. If that turns out to be false, we’ll leave.”

He signed off the show, called “Tucker on Twitter,” by promising to be back with “much more, very soon.”

Fox management took Mr. Carlson off the air after concluding that he was too much of a liability. The two sides have not reached an agreement about the terms of a departure, including what kind of programming he could continue to make. Anchors like Mr. Carlson usually also have terms in their contracts that prevent them from making unsanctioned media appearances.