MOSCOW - The deadly attack on a concert hall near Moscow was conducted by Islamist militants, but the shooting fits in a wider campaign of intimidation by Ukraine, Russian President Vladimir Putin said on Monday.

Putin said the investigation underway into the mass shooting had to be conducted "professionally, objectively and without political bias" to determine who was behind the attack.

"The question that arises is who benefits from this?" Putin told a Kremlin meeting to discuss a response to the shooting.

"This atrocity may be just a link in a whole series of attempts by those who have been at war with our country since 2014 by the hands of the neo-Nazi Kyiv regime.

"We know by whose hand the crime against Russia and its people was committed. But what is of interest to us is who ordered it." Ukraine denies any involvement in the attack.

Alexander Bastrykin, chairman of the Investigative Committee, Russia's main criminal investigation organ, told the meeting the death toll in Friday's shooting had climbed to 139.

  - Reuters