Russia on Monday challenged assertions by the United States that the Islamic State militant group orchestrated a gun attack on a concert hall outside Moscow that killed 137 people and injured 182 more, accusing Washington of covering for Ukraine.

In the deadliest attack inside Russia for two decades, four men burst into the Crocus City Hall on Friday night, spraying people with bullets during a concert by the Soviet-era rock group Picnic.

Four men, at least one a Tajik, were remanded in custody on terrorism charges. They appeared separately, led into a cage at Moscow's Basmanny district court by Federal Security Service officers.

Islamic State has claimed responsibility for the attack, a claim that the United States has publicly said it believes, and the militant group has since released what it says is footage from the attack. U.S. officials said they had warned Russia of intelligence about an imminent attack earlier this month.

President Vladimir Putin has not publicly mentioned the Islamist militant group in connection with the attackers, who he said had been trying to escape to Ukraine.

Putin said some people on "the Ukrainian side" had been prepared to spirit the gunmen across the border. Ukraine has denied any role in the attack and President Volodymyr Zelenskiy has accused Putin of seeking to divert blame for the attack by mentioning Ukraine.

Russia's Foreign Ministry spokeswoman, Maria Zakharova, called into question U.S. assertions that Islamic State, which once sought control over swathes of Iraq and Syria, was behind the attack.

In an article for the Komsomolskaya Pravda newspaper, she said the United States was evoking the "bogeyman" of Islamic State to cover its "wards" in Kyiv, and reminded readers that Washington had supported the "mujahideen" fighters who fought Soviet forces in the 1980s.

Two U.S. officials said on Friday that the United States had intelligence confirming Islamic State's claim of responsibility.

Kremlin spokesman Dmitry Peskov later told reporters that Russia could not comment on the Islamic State claim while the investigation was ongoing, and would not comment on the U.S. intelligence, saying it was sensitive information.

Video shows part of suspect's ear being cut off

Putin said 11 people had been detained, including the four suspected gunmen, who he said had fled the concert hall and made their way to the Bryansk region, about 340 km (210 miles) southwest of Moscow, to slip across the border to Ukraine.

Unverified videos of the suspects' interrogations circulated on social media. One of the suspects was shown having part of his ear cut off and stuffed into his mouth.

One man, a Tajik named Dalerdzhon Mirzoyev, leaned against the glass cage as the terrorism charge was read out. Saidakrami Rachabalizoda, his ear in bandages, remained sitting.

Muhammadsobir Fayzov, appeared in gaping hospital clothes and sat in a medical chair, his face covered in cuts. Shamsiddin Fariduni, his face bruised, stood.

The Kremlin's Peskov left a journalist's question about the treatment of the detainees unanswered.

Putin ordered a full-scale invasion of Ukraine in February 2022, triggering a major European war after eight years of conflict in eastern Ukraine between Ukrainian forces on one side and pro-Russian Ukrainians and Russian proxies on the other.

The U.S. and its European allies have supported Ukraine, extending billions of dollars of money, weapons and intelligence in a bid to defeat Russian forces.

The French government said late on Sunday it was raising its terror alert warning to the highest level after the shootings in Moscow.