Russian security officials are hunting three women they believe are potential suicide bombers. One of them is believed to be in Sochi, where the Winter Olympics will begin next month.

Police have distributed leaflets, warning of the three potential suicide bombers. The one thought to be in Sochi is a 22-year-old widow of an Islamic militant.

Russia has blamed the so-called black widows of slain insurgents for previous suicide attacks. Security authorities have mounted an extensive operation in Sochi, but concerns persist that soft targets outside the Olympic venues, such as buses and tourist facilities, are vulnerable to attack.