(Reuters) - Russian President Vladimir Putin authorised a military operation in eastern Ukraine on Thursday in what could be the start of war in Europe over Russia's demands for an end to NATO's eastward expansion.

       Shortly after Putin spoke, a Reuters witness heard the sound of what appeared to be explosions in the distance from the capital, Kyiv.

       COMMENTS:

       U.S. SENATOR MARK WARNER, CHAIRMAN OF SENATE INTELLIGENCE COMMITEEE, WASHINGTON:

       "For more than 70 years, we have avoided large-scale war in Europe. With his illegal invasion of Ukraine, Vladimir Putin has tragically brought decades of general peace to an end. Now the U.S. and our NATO allies must stand united and resolute against Putin's efforts to renew the Russian empire at the expense of the Ukrainian people."

       (Reporting by Andrea Shalal; Compiled by Christopher Cushing)