Liam Hemsworth allegedly cheated on his fiance with actress January Jones in late February and an eyewitness relayed details about the affair where the stars "made out" to Life & Style.

The 23-year-old Hunger Games actor, engaged to Miley Cyrus, and Mad Men star were spotted getting close at a Grey Goose pre-Oscar party on Feb. 23.

"They went to a private house party, and right in the middle of the party, January was clinging to Liam, saying, 'You're so handsome.' He kept saying, 'We can't do this here.' " the source said. "Then they left holding hands and went out in the hallway and were making out. She was wasted. It wasn't good."

The insider added that upon hearing news of Hemsworth and Jones being together at the party, Cyrus was "freaking out." The 20-year-old singer/actress has also been telling her friends she trusts Hemsworth but "if she finds out the cheating claims are true, the engagement is over."

Hemsworth and Jones were also photographed leaving the Grey Goose party together in the same car and an eyewitness said, "it looked like they had some hot chemistry."

Cyrus and Hemsworth reportedly cancelled their wedding recently because of hectic work schedules. However, a source told Us Weekly the couple might be headed toward a breakup since Hemsworth is having second thoughts about getting married because of Cyrus constantly working.

"Miley put her career on hold to spend all her time with Liam. Now that she is back recording her fourth album, he's not happy. She says she's already over it," the source said. "Work has put a stress on their relationship."