WASHINGTON (Reuters) - The White House on Saturday said that Russia could launch an attack against Ukraine at any time and said U.S. President Joe Biden will convene a National Security Council meeting on Sunday to discuss the situation.

       Russia's strategic nuclear forces held exercises overseen by President Vladimir Putin on Saturday, and Washington accused Russian troops massed near Ukraine's border of advancing and being "poised to strike."

       (Reporting by Jeff Mason; Writing by Daphne Psaledakis; Editing by Jonathan Oatis)