Reuters, Moscow,the Daily Star, Bangladesh/ Photo by CNBC

At least 40 people were killed and more than 100 injuredwhen five gunmen dressed in camouflage opened fire with automatic weapons atpeople at a concert in the Crocus City Hall near Moscow, in one of the worstsuch attacks on Russia in years.

Russian media reported a second blast at the venue and there were reportsthat some of the gunmen had barricaded themselves somewhere in the building.

The shooting appeared to have begun at a concert of the band"Picnic".

"Suddenly there were bangs behind us - shots. A burst of firing - I donot know what," one witness who asked not to be named told Reuters.

"A stampede began, everyone ran to the escalator," the witnesssaid. "Everyone was screaming, everyone was running."

The shooting began days after President Vladimir Putin wasre-elected for a new six-year term and as Russia is prosecutinga war with Ukraine.

State news agency TASS cited Russia's Federal Security Service (FSB) assaying 40 people had been killed and more than 100 wounded.

The FSB said all necessary measures were being taken.

It was not immediately clear who the gunmen were. Russian news agenciessaid 70 ambulance crews had been sent to the scene.

"A terrible tragedy occurred in the shopping center Crocus Citytoday," Moscow Mayor Sergei Sobyanin said. "I am sorry for the lovedones of the victims."

Russia tightened security at airports and stations and across thecapital - a vast urban area of over 21 million people. Putin has yet to commentin public.

Russia's foreign ministry said it was a "bloody terrorist attack".

"The entire world community is obliged to condemn this monstrouscrime," Foreign Ministry Spokeswoman Maria Zakharova said. "Allefforts are being thrown at saving people."

SHOOTING PEOPLE

In one unverified video posted on social media, men with automatic weaponswere shown firing repeatedly at screaming civilians, including women, who werecowering below what looked like an entrance sign to "Crocus CityHall".

Other video footage showed a number of people lying motionless in pools ofblood outside the hall. Reuters was unable to immediately verify the footage.

Another video showed the attackers shooting at people in the concert hall.

The White House said on Friday that images of shooting in the Russiancapital were hard to watch.

"The images are just horrible and just hard to watch and our thoughtsobviously are going to be with the victims of this terrible, terrible shootingattack," White House spokesman John Kirby said.

The U.S. embassy in Russia warned earlier this month that"extremists" had imminent plans for an attack in Moscow.

It issued its warning several hours after the FSB, the main successor to theSoviet-era KGB, said it had foiled an attack on a synagogue in Moscow by a cellof the militant Sunni Muslim group Islamic State.