Mel Gibson will sue Oksana Grigorieva for extortion if she dares sue him ... this according to sources close to Gibson.
As TMZ first reported, Oksana still plans to file a lawsuit against Mel, alleging battery, emotional distress and defamation.
But sources close to Gibson tell us he will wage legal war if Oksana files -- and will NOT settle out of court. In fact, we're told Mel will gladly file his own lawsuit against the woman he says put a $15 million price tag on the tapes in return for keeping them secret.
Looks like this battle has just begun.