Russian investigators claim they have evidence of a link between the perpetrators of last week's terror attack on a concert hall and "Ukrainian nationalists."

Svetlana Petrenko, the spokesperson for the Russian Investigative Committee, made the announcement in Moscow on Thursday.

Petrenko said the investigation has confirmed that the attackers received "significant amounts of money and cryptocurrency from Ukraine," which were used in preparing for the crime.

Authorities have already charged four men in the attack. Petrenko said investigators analyzed technical devices they seized from the suspects and found financial transactions connecting them to Ukraine.

The Islamic State group has claimed responsibility for the attack, and Ukraine has denied any involvement. Ukrainian military intelligence chief Kyrylo Budanov called the allegations "nonsense."

Budanov added that a Russian intelligence group in Syria had been aware of the plot as early as February 15. He said the information was "forwarded to Moscow."

John Kirby, the White House national security spokesperson, said US government officials had sent a written warning to Russian security services on March 7. He added that it is "abundantly clear" the Islamic State group was "solely responsible."