Hailey Baldwin has convinced Justin Bieber to stay in a monogamous relationship with her, giving him an ultimatum to end things with Kourtney Kardashian, Us Weekly reported, citing sources. Earlier reports concluded that Bieber and Baldwin were dating after the 19-year-old model was seen getting close to Bieber on vacation in Anguilla.

“She demanded he stop stringing her along,” a source close to Bieber said, according to Us Weekly, adding: “She said he would lose her if he didn’t man up and show he was serious about her.” The report said that Baldwin has given Bieber an ultimatum to end things with Kardashian, who was said to be casually dating the "Sorry" singer.

The source also said, referring to Baldwin: “Justin has never had a woman act like that before,” adding: “He couldn’t say no. So he’s been seeing her again and is with her all the time.”

The newly linked couple posted several photos with each other on Instagram from their vacation, spurring speculation about what his former girlfriend Selena Gomez and Kardashian would think of Baldwin.

Bieber was recently asked on a radio show to comment about his rumored casual relationship with Kardashian and he said: “I’m being used, man. What can I say?!”

The comment hurt Kardashian and a report last week by Hollywood Life said, citing a source, that she felt as if she was “stabbed in the back” after the comment and wanted the “What Do You Mean” singer to apologize.

“Kourtney wants a major explanation and apology. She’s not the one who leaked their romance to the public. She feels like she’s being punished and it just goes to prove her initial instinct — that Justin is too young and too immature for her. She’s kind of beating herself up for ever getting involved with him in the first place, and not listening to her gut. She really regrets the whole thing,” a source close to Kardashian told Hollywood Life, adding: “She has been nothing but respectful towards him and their relationship. She has been so protective of him and now she feels completely stabbed in the back.”