WASHINGTON (AP) — President Joe Biden is vowing to make it “very, very difficult” for Russian President Vladimir Putin to take military action in Ukraine, saying his administration is putting together a comprehensive set of initiatives to curb Russian aggression. The president offered the measured warning to Putin amid growing concern about a Russian buildup of troops on the Ukrainian border and increasingly bellicose rhetoric from the Kremlin. Biden told reporters he is putting together “the most comprehensive and meaningful set of initiatives to make it very, very difficult for Mr. Putin to go ahead and do what people are worried he may do.”