President Trump on Saturday followed up what he called a "great" meeting with German Chancellor Angela Merkel by blasting her country for what he said were unpaid bills for NATO defense.On Twitter, Trump reiterated a point he made at his joint news conference Friday with Merkel that many NATO nations owe money to the alliance and the U.S. because they haven't paid their fair share.This time, he chastised Germany for not paying enough "for the powerful, and very expensive, defense" NATO and the U.S. provide.Despite what you have heard from the FAKE NEWS, I had a GREAT meeting with German Chancellor Angela Merkel. Nevertheless, Germany owes.....&mdash; Donald J. Trump (@realDonaldTrump) March 18, 2017 ...vast sums of money to NATO &amp; the United States must be paid more for the powerful, and very expensive, defense it provides to Germany!&mdash; Donald J. Trump (@realDonaldTrump) March 18, 2017 Trump didn't single out Germany for NATO bills as he stood next to Merkel and addressed reporters' questions on Friday at a news conference in which he also doubled down on his unsubstantiated accusations that President Obama had ordered surveillance of him during the 2016 campaign."Many nations owe vast sums of money from past years, and it is very unfair to the United States," Trump&nbsp;said. "These nations must pay what they owe."He then thanked Merkel for Germany's commitment to increase its defense spending.Merkel said Germany had&nbsp;increased its defense spending by 8% last year and was committed to reaching NATO's goal for each member nation to spend&nbsp;2% of total economic output on defense by 2024.