At least 133 people were killed after a group of armed men in combat fatigues stormed into a concert hall in Moscow and unleashed gunfire and detonated explosives among the crowd on Friday (local time), Russian media reported.
The Russian intelligence agencies detained 11 people, including four 'terrorists', who they claimed were 'directly' involved in the attack.
Russian agencies appeared to suggest the attack was linked to Ukraine even though the Islamic State group claimed responsibility in a statement.
The terror group took responsibility for the attack in a short statement published by ISIS-affiliated news agency Amaq on Telegram on Friday.
However, ISIS has not provided any evidence to support the claim.
The four suspects were stopped in the Bryansk region of western Russia, "not far from the border with Ukraine", Russia's Investigative Committee said.
They planned to cross the border into Ukraine and "had contacts" there, state news agency Tass said, citing Russia's FSB.
Al Jazeera cited a video posted on social media that showed huge plumes of black smoke rising over the building.
Another video footage from the scene showed a large plume of black smoke billowing from the burning building, which appeared engulfed in flames.
Around 100 people were evacuated from the building by firefighters, TASS reported.
More than 50 ambulance teams were sent to Krasnogorsk, a town in the Moscow region, to provide medical care to all those in need, the Moscow Region Ministry of Health told RIA Novosti.
The attack came just days after Putin cemented his grip on power in a highly orchestrated electoral landslide.
The attack was the deadliest in Russia in years and came as the country's fight in Ukraine dragged into a third year.
Moscow Mayor Sergei Sobyanin has expressed condolences for the people whose loved ones have been killed and called the incident a "terrible tragedy", as reported by CNN.
"Today a terrible tragedy occurred in the Crocus City center. My condolences to the loved ones of the victims. I gave orders to provide all necessary assistance to everyone who suffered during the incident," Sobyanin said in a statement.
Meanwhile, Russian foreign ministry spokesperson Maria Zakharova has termed the incident a "monstrous crime" and has also called on the world to condemn the incident.
While, the White House has condemned the attack, but has clarified that there is "no indication" of any Ukrainian involvement in the attack.
The White House said that the images of shooting in the Russian capital were "horrible and just hard to watch," Al Jazeera reported.
"There is no indication at this time that Ukraine or Ukrainians were involved in the shooting," National Security Council spokesman John Kirby told reporters. "I would disabuse you at this early hour of any connection to Ukraine."
Moscow has questioned Washington's firm belief of Ukraine not being involved and has asked the US to share any information it has about the attack.
"The White House said it sees no signs that Ukraine or Ukrainians are involved in the terrorist attack in Moscow," Foreign Ministry spokeswoman Maria Zakharova said.
"On what basis do officials in Washington draw any conclusions in the midst of a tragedy about someone's innocence?" she added.
She said that if Washington had information, it should be shared and that if Washington had no information, it should not be talking in such a way.
