Upcoming Paris Olympics could become 'premium terrorist target', says NGO

"I worry about the Paris Olympics ... They would be a premium terrorist target," the adviser told The New York Times newspaper.

The concerns come as counterterrorism officials said the recent terrorist attack in Russia was more sophisticated than others over a period of time: there was a greater level of planning and an ability to infiltrate into local extremist networks, the newspaper reported.

Among the main forces that could motivate such attacks during the upcoming Olympics, a senior Western intelligence official cited the existence of dormant terrorist cells in Europe and images of the war in the Gaza Strip circulating on the internet.

The January UN report on terrorism said some people of North Caucasian and Central Asian origin who traveled to Europe from Afghanistan or Ukraine could present an opportunity for a branch of the Islamic State terrorist group (banned in Russia) known as ISIS-Khorasan to launch attacks against the West, according to the report.

US authorities blame ISIS-Khorasan for the attack on Crocus City Hall.

A shooting occurred on Friday evening at the Crocus City Hall concert venue in the city of Krasnogorsk, outside Moscow, followed by a massive fire.

Several men wearing camouflage broke into the music hall, shooting people point-blank and throwing incendiary bombs, a Sputnik correspondent reported. Russian authorities said that at least 137 people were killed in the attack.

Eleven people have been detained in connection with the attack, including four who were directly involved, the Russian Federal Security Service (FSB) said. All four suspected shooters were apprehended in Russia's Bryansk Region, which borders Belarus and Ukraine, the FSB added.

The Olympic Games in Paris will take place from July 26 to August 11.