The 25-year-old "The Hunger Games" star got cozy with Monroe, 22, at a party thrown by director Roland Emmerich, reported Us Weekly Magazine.
"His hand was on her butt as she was grinding into him and they were getting cozy at the bar," a source said.
The party included guests such as Vivica A Fox, Bill Pullman, and Lance Bass.
"Liam was draped all over her. It was so PDA and in-your-face," the source added.
The rumoured pair however have not made anything official yet.
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"They're not official. But their chemistry is ridiculous," an insider said.
"Independence Day: Resurgence" is scheduled to release in June 2016.