The allegations are part of a lawsuit filed by the 25-year-old hooper in Los Angeles.

Oubre -- a 6-year NBA vet who was a former 1st-round draft pick -- claims his ex, Makena LeDuff, contacted him on January 11, 2021, and demanded a large sum of money ... or she would retaliate against him.

"Makena demanded that Kelly her pay her nearly $3 million or she would make public her false, scurrilous, and extortionate allegations by the end of the week," the lawsuit reads.

The suit claims LeDuff contacted Oubre and threatened to say he was engaged in "criminal conduct" and make other false allegations ... all "crafted to cause maximum and irreparable harm to Kelly, his reputation, and his career."

If this case sounds familiar ... well, that's 'cause it is.

Oubre first filed suit against LeDuff back in 2019 .... saying she became an absolute nightmare after they broke up. He claims she stole his 2 dogs, Soul and Saint, slashed his tires and refused to leave the house he was renting in Beverly Hills.

Kelly says he still hasn't gotten his dogs back ... and now the situation has escalated with the threats. So, his attornies amended their initial complaint ... and now they're seeking punitive damages against LeDuff.

That's not all -- Oubre also claims Makena threatened to hurt him with a thinly veiled threat she posted on social media back in August 2019.

"Makena posted a photo on her Instagram account, where she intimated that she would take physical action against Kelly, Kelly’s friends, or anyone else that tried to enter the house to help Kelly to retrieve his things," the lawsuit claims.

The suit continues ... "Specifically, the photo showed Makena in the backyard of the Beverly Hills house, wearing one of Kelly’s favorite hats, while at the same time brandishing a rifle. In the caption, Makena wrote, 'Exercising ALL my rights.'"