Donald Trump and Russian government authorities lashed out Wednesday after reports that U.S. intelligence agencies informed the president-elect of unsubstantiated allegations that Kremlin operatives have compromising material on the president-elect and engaged in an extensive conspiracy with members of his team in order to help get him elected.

Mr. Trump, at a midday news conference, his first since being elected, also for the first time said be believes Russia was behind cyberattacks aimed at influencing the November election....