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

Flames leapt into the sky and plumes of black smoke rose above the venue, Reuters pictures and video showed.

Russian media reported a second blast at the venue and there were reports that 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 do not know what," one witness who asked not to be named told Reuters.

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

The shooting began days afterPresident Vladimir Putinwas re-elected for a new six-year term and as Russia is prosecuting awarwith Ukraine.

State news agency TASS cited Russia's Federal Security Service (FSB) as saying 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 agencies said 70 ambulance crews had been sent to the scene.

"A terrible tragedy occurred in the shopping center Crocus City today," Moscow Mayor Sergei Sobyanin said. "I am sorry for the loved ones of the victims." (Reuters, , excluding headline)