Forty people were killed in a shooting at the Crocus City Hall concert centre near Moscow on Friday, state news agency TASS reported, citing information from Russia's Federal Security Service (FSB).
More than 100 people had been injured in the same incident, TASS reported.
The roof of Crocus City Hall, the flame-engulfed concert centre near Moscow where gunfire and explosions were heard on Friday night, is collapsing, Russian state-owned news agency RIA reported.
The White House on Friday said it did not encourage or enable Ukraine to strike inside Russia.
White House spokesperson John Kirby declined to comment on a Financial Times report that the United States had urged Ukraine to halt strikes on Russian energy infrastructure, warning Kyiv that drone strikes risk provoking retaliation and driving up global oil prices.
But he said Washington had a longstanding policy of not encouraging Ukrainian forces to conduct strikes inside Russia.