5:38 PM PT -- Offset just responded to CB's challenge ... saying "coke head don't want" smoke.
Chris Brown is going after Offset as their beef over 21 Savage continues to escalate ... with CB challenging Offset to a fight, and telling him to suck his d***.
Chris went NUCLEAR after the Migos rapper called him "lame" for posting a meme poking fun at 21 Savage for being from the United Kingdom.
Breezy launched an explicit series of insults at Offset, saying ... "F*** YOU LIL BOY ... SENSITIVE ASS N**** ... IF YOU A REAL MAN FIGHT ME. Oh and another thing, SUCK MY D***!!!!"
CB came back for seconds, because he hurled more insults ... adding this on top of his prior barbs, "If he don't get his EARTH WIND AND FIRE pootytang space suits WEARING CAP ASS LIL BOY da f*** out my comments and off my d***."
As you know, Offset and 21 have been close for years ... and it seems Offset isn't taking crap from anyone making fun of his boy for getting detained by ICE, a saga that's revealed 21 is actually from the UK and not from Atlanta.
As for the meme Breezy posted that led Offset to call him out ... it's a 21 Savage music video with a British-sounding rapper doing a voice-over.