9:06 AM PT -- Sources with direct knowledge tell TMZ ... Krayzie Bone is going into a second surgery soon in the hopes of stopping some internal bleeding, which is at the root of his health troubles.

We're told this all started Friday when KB started coughing up blood, after which he checked himself into an L.A.-area hospital. After some tests, our sources tell us docs discovered a bleeding artery in one of his lungs ... and they operated on him.

Afterward, we're told Krayzie was placed in an induced coma -- with the idea being that would help the healing process -- but our sources say the issue wasn't resolved, and that the artery continues to bleed in his lung. Now, they're going back to try and fix it for good.

Our sources tell us his family members have been the only ones allowed to see him, and they're at his side now. Krayzie's team, we're told, is meeting today to get more updates.

Krayzie Bone is reportedly fighting for his life in a hospital -- a scary update that has much of Hollywood sending their thoughts and prayers his way.

The Bone Thugs-N-Harmony rapper is said to be on a ventilator as a result of surgery to deal with a respiratory issue he's been battling, which saw him coughing up blood and requiring to go under the knife immediately ... this according to AllHipHop.com.

The outlet reports KB's initial hospitalization stems from a diagnosis of sarcoidosis -- which causes inflammatory cells to form in different parts of the body. After undergoing a procedure to stop internal bleeding doctors detected, sources tell AHH that his condition worsened.

The news appears to have been confirmed by one of Krayzie's own BTNH members, Bizzy Bone -- who posted on social media this weekend ... asking everyone to keep KB in their thoughts. That, of course, spurred a lot of people to react ... including some big names.

Among the celebs who threw up messages of support ... Juicy J, LeBron James, DJ Paul, and others. Of course, tons of hip hop fans at large are pouring out with tributes as well.

Krayzie Bone's family is reported to be seeking privacy and hasn't expanded on where things stand -- but word is ... he's currently sedated and remains in Los Angeles-area ICU.

His contributions to music can't be overstated -- in addition to being a part of one of the biggest hip hop groups to emerge from the '90s, with an incredibly unique style, Krayzie has gone on to launch his own record label, apparel line and podcast network ... to name a few accomplishments. He's also won a couple Grammys in his day as part of Bone Thugs.

The guy isn't even all that old ... he's only 50. Here's hoping he can pull through. 🙏