Kim Kardashian West has addressed allegations that Tristan Thompson cheated on her sister Khloe while she was pregnant with their daughter True, saying it is "a really sad situation."

Kim's younger sister Khloe gave birth to her first child with NBA star Thompson on April 12, shortly after reports emerged that he had been unfaithful.

In a preview of an interview on The Ellen DeGeneres Show that will air in the US on Monday, DeGeneres asked Kim, "Everybody was heartbroken for her (Khloe), literally days before she gives birth, her boyfriend is seen on video cheating, you have opinions about that?"

Kim replied, "Poor Khloe. Like, I don't even know how to describe it besides it's just so f***ed up."

"We really were rooting for Khloe. And we still are. She's so strong and she's doing the best that she can. It's a really sad situation, all over."

She added: "I kind of made this rule with my brother (Rob Kardashian) - if there's a baby involved, I'm going to keep it cute and keep it classy and not talk too bad.

"Well, I'm going to try not to say anything so negative because one day, True is going to see this and you know, it's so messed up."

Taking to Instagram after the birth of True, Khloe said that she and Tristan were "overwhelmed" by their love for their daughter.

"Our little girl, True Thompson, has completely stolen our hearts and we are overwhelmed with LOVE," she captioned an image of pink flowers and balloons.

"Such a blessing to welcome this angel into the family! Mommy and Daddy loooooove you True!"