Outgoing CIA Director John Brennan on Sunday issued a stern parting rebuke to Republican Donald Trump days before he assumes the U.S. presidency, advising him not to absolve Russia for its recent actions and warning him to watch what he says. In return, Trump responded by slamming Brennan and criticizing U.S. policies under Obama administration.Brennan's comments, in an interview on "Fox News Sunday," laid bare the simmering tensions between the president-elect and the intelligence community he has criticized and is on the verge of commanding.The CIA director said Trump needs to be mindful about his off-the-cuff remarks once he takes the oath of office on Friday, alluding to his penchant for making broad pronouncements on Twitter."Spontaneity is not something that protects national security interests and so therefore when he speaks or when he reacts, just make sure he understands that the implications and impact on the United States could be profound," Brennan said."It's more than just about Mr. Trump. It's about the United States of America.""Outgoing CIA Chief, John Brennan, blasts Pres-Elect Trump on Russia threat. Does not fully understand." Oh really, couldn't do... Trump said on his Twitter account, and continued by saying that the 'red line' was crossed in Syria, Crimea, and Ukraine, as Russia built nukes.