Miley Cyrus, Katy Perry -- GIRL WAR Over Kiss!

Miley Cyrus and Katy Perry are calling each other dirty bitches ... in a full-on feud that erupted over that kiss at Miley's L.A. concert last month.

You may recall ... Miley went in for the kiss as Katy sat in the front row. It looked like Miley was going in for some tongue, but Katy pulled away.

Now the feud.

Katy went on Australian TV and explained, "I just walked up to her to give her like a friendly girl kiss, you know, as girls do." Perry then explained how she was stunned when Miley went in for the kill, adding, "God knows where that tongue has been. We don't know. The tongue is so infamous!"

If Katy meant it as a joke, Miley isn't taking it that way, firing back on Twitter, "Girl if ur worried abt where tongues have been good thing ur ex boo is ur EX BOO cause we ALL know where THAT [tongue] been."