Russia's FSB security service has said that four people arrested Sunday in a foiled "terror" plot had provided money and arms for the deadly attack on a Moscow concert hall last month.

More than 140 people were killed when gunmen stormed the Crocus City Hall venue on March 22 before setting the building on fire in the most fatal attack in Russia for two decades.

The FSB said in a statement on Monday that it had arrested a group of four a day earlier in the southern Dagestan region who "were directly involved in the financing and supply of terrorist means to the perpetrators of the terrorist act carried out on 22 March in the Crocus City Hall in Moscow."

On Sunday, Russia's national anti-terrorism committee said it had apprehended three people who were "planning to commit a series of terrorist crimes."

The FSB said Monday that four foreign citizens had been arrested in the operation in the regional capital Makhachkala and the nearby town of Kaspiysk.

The Interfax news agency cited an FSB video showing one of the detained men saying: "I took weapons to them, these guys who attacked Crocus City Hall. I took them weapons from Makhachkala."

Russian authorities had previously announced the arrests of 12 people they say are connected to the attack -- including the four suspected gunmen, who have been identified as Tajik citizens.

The Islamic State (IS) has claimed responsibility for the massacre, the most deadly it says it has ever carried out on European soil, though President Vladimir Putin has talked up a Ukrainian and Western connection.

Kyiv and the West have repeatedly denied any involvement and accused Moscow of "exploiting" the tragedy.

Dagestan is a Muslim-majority region in Russia's southern Caucasus region.

The FSB has come under scrutiny over its failure to thwart the attack despite private and public warnings by the US intelligence community that "extremists" were planning an "imminent" attack on "large gatherings" in Moscow.

The agency regularly announces it has foiled alleged "terrorist cells", but in recent months has mainly announced the arrests of what it calls pro-Ukrainian saboteurs planning attacks on Russian military sites and infrastructure.