The White House said on Friday it had no initial indication that Ukraine, which is fighting an invasion by Russia, was involved in a "terrible" attack on a concert hall in Moscow.
National Security Council spokesman John Kirby said the United States was still gathering information after Russian officials said gunmen opened fire in a "terrorist" attack, leaving dead and wounded.
"There is no indication at this time that Ukraine, or Ukrainians, were involved in the shooting," Kirby told reporters. "I would disabuse you at this early hour of any connection to Ukraine."
Kirby also offered condolences to the victims of the attack, news of which broke minutes before the daily White House briefing.
"The images are just horrible and hard to watch, and our thoughts obviously are going to be with the victims of this terrible, terrible shooting attack," he said.
"You've got to recognize that there's some moms and dads, brothers and sisters and sons and daughters that haven't gotten the news yet. And this is going to be a tough day."
The U.S. Embassy had warned on March 7 that "extremists" were planning something with the Russian presidential election just days away, but Kirby said Washington had no prior knowledge of the attack.
"I'm not aware of any advanced knowledge that we had of this," adding that he did not believe the earlier warning referred to Friday's attack.
The U.S. State Department has now warned all Americans to avoid concerts or shopping malls and to stay in place following the latest violence, he added.
