MOSCOW (Reuters) -Russian investigators said on Thursday they had found proof that gunmen who killed more than 140 people at a concert last week were linked to “Ukrainian nationalists”, an assertion immediately dismissed by the United States as baseless propaganda.

Russia has said from the outset that it believes Ukraine was linked to the attack, even though Kyiv has denied it and the militant group Islamic State has claimed responsibility.

In a statement, Russia’s Investigative Committee said for the first time that it had uncovered proof of a Ukrainian connection. While it described the nature of the alleged evidence, it did not publish it.

“As a result of working with detained terrorists, studying the technical devices seized from them, and analysing information about financial transactions, evidence was obtained of their connection with Ukrainian nationalists,” it said.

It said there was “confirmed data” that the attackers had received significant amounts of cash and cryptocurrency from Ukraine. Another suspect involved in “the terrorists’ financing scheme” had been detained, the committee said.

Minutes later, White House national security spokesman John Kirby described the Russian allegations as “nonsense and propaganda” and said that Islamic State was solely responsible for the attack.

The United States had publicly warned earlier in March that it had intelligence that “extremists” were planning an imminent attack in Moscow. Kirby said it had also provided written warning to Russian security services.

“In fact, the United States tried to help prevent this terrorist attack and the Kremlin knows this,” he said.

U.S. officials say they believe it was Islamic State Khorasan, the network’s Afghan branch, that was responsible for the concert shooting. Russia says the fact that the U.S. was able to name the alleged perpetrator so fast is suspicious.

The head of Russia’s FSB security service said earlier this week, again without providing evidence, that he believed Ukraine, along with the U.S. and Britain, were involved.

Western security analysts say the attack raised questions about the resourcing and priorities of Russian intelligence agencies that have been heavily focused on the Ukraine war and the need to stamp out opposition to it within Russia.

Eleven people were arrested in the first 24 hours after last Friday’s shooting and eight of these, including the four suspected gunmen, have been placed in pre-trial detention. Seven are from the Central Asian state of Tajikistan and the other from Kyrgyzstan.

(Reporting by Reuters in Moscow and Jonathan Landay, Jarrett Renshaw and Rami Ayyub in Washington; writing by Mark Trevelyan, Editing by Angus MacSwan)