"Coming on the heels of the recent attack on the Moscow-Saint Petersburg train, this significant loss of life and multiple casualties are especially painful," said National Security Council spokesman Mike Hammer in a statement.
At least 109 people were killed and more than 130 others injured as fire ripped through a nightclub in Russia's Urals city of Perm late on Friday.
The Russian authorities earlier ruled out terrorist attacks as a possible cause of the incident, saying bomb experts found no evidence of explosive devices.
Russian President Dmitry Medvedev has declared a national day of mourning for the victims on Monday when Russian national flags will be flown at half mast and TV, radio stations and cultural establishments will be required to cancel all entertainment events.
The blast occurred exactly a week after a train traveling from Moscow to St. Petersburg was derailed by a homemade bomb. Twenty-seven people were killed in the terrorist attack.
WASHINGTON, Dec. 5 (Xinhua) -- The United States said on Saturday that it was "deeply saddened" for the deadly fire in the Russian Urals city of Perm, in which at least 109 people were killed.