The Moscow subway needs some 300-400 sniffer dogs to ensure security, the head of the Moscow metro said Thursday.

Dmitry Gayev said the animals are useful, not just for being able to sniff out explosives, but also for bringing psychological comfort to passengers. He also mentioned a system operating in other countries in which the reaction of passengers to sniffer dogs is monitored.

"Specially trained people look into cameras to see how passengers react to the dogs, and decide from the reaction whether to check the person or not," he said.

Security is being stepped up in Russia after two terrorist attacks on the Moscow subway on Monday killed 39 people and injured dozens more.

On Wednesday, a further two explosions in the town of Kizlyar, near Dagestan's border with Chechnya in Russia's North Caucasus, killed 12 people.

MOSCOW, April 1 (RIA Novosti)