The gunmen, wearing combat fatigues, also used explosives, causing a huge blaze at the Crocus City Hall, the reports said.

At least 40 people were killed in the attack and more than 100 wounded, Russian state news agency TASS reported, citing Russia's Federal Security Service (FSB).

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About 100 people were evacuated from the basement of the concert hall near Moscow, but there were still people on the roof.

About 50 ambulance crews rushed to the scene, local news agencies said.

One news agency said the roof of the flame-engulfed building was collapsing.

It comes days after Vladimir Putin secured his fifth term as Russia's president following an electoral landslide.

Videos showed flames and plumes of black smoke rising into the sky.

In other videos posted by Russian media and Telegram channels, extended rounds of gunfire could be heard while two men with rifles were moving through the mall.

Moscow Mayor Sergei Sobyanin said: "A terrible tragedy occurred in the shopping centre Crocus City today."

He added: "I am sorry for the loved ones of the victims."

White House national security adviser John Kirby said he could not yet speak about all the details but that "the images are just horrible. And just hard to watch".

"Our thoughts are going to be with the victims of this terrible, terrible shooting attack," Mr Kirby said.

"There are some moms and dads and brothers and sisters and sons and daughters who haven't gotten the news yet. This is going to be a tough day."

He said there was "no indication at this time that Ukraine, Ukrainians were involved".

But Russia said if the US knew for sure Ukraine was not involved then it should share any information it had.

Russian 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 said if Washington had information it should be shared and if it had no information it should not be talking in such a way.

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