U.S. intelligence sources said they warned Russia repeatedly that extremists were planning to attack large gatherings ahead of last week's attack on a Moscow concert hall that killed over 140 people.

National Security Adviser John Kirby said a written warning was passed on March 8 to Russian security services about a possible attack, which is customary practice with foreign governments when threats are involved.

Kirby also responded to claims that Ukraine was involved in the attack, saying that was absolutely not true.

"It is abundantly clear that ISIS (Islamic State) was solely responsible for the horrific attack in Moscow last week," Kirby said. "In fact, the United States tried to help prevent this terrorist attack and the Kremlin knows this."

"Nonsense and propaganda"

Russia's Investigative Committee claimed after the attack that the four gunmen who carried it out were linked to Ukrainian nationalists and had been given cash and cryptocurrency from Ukraine as payment.

Kirby called those allegations "nonsense and propaganda."

Ukrainian presidential adviser Mykhailo Podolyak denied the accusations in a post on X.

"Ukraine certainly has nothing to do with the shooting/explosions in the Crocus City Hall (Moscow Region, Russia). It makes no sense whatsoever," Podolyak wrote on Mar. 22.

Horrific attack

The gunmen, who are from Tajikistan, appeared in court Sunday with cuts and bruises and swollen faces, the Guardian reported.

Putin said he would not visit the site of the deadly attack because he would interfere with the work being done there.

In addition to shooting the victims, the gunment also set the concert hall on fire.

"The terrorists used a flammable liquid to set fire to the concert hall's premises, where spectators were located, including wounded," Russia's Investigative Committee said.

Helicopters dropped water on the blaze, but it took 10 hours to contain the fire once it had started.

The rock band Picnic was about to perform a concert at the venue when the attack happened.