Moscow (CNN) -- Russian authorities are investigating six suspicious deaths and at least one car explosion this week about 240 kilometers (150 miles) from the site of next month's Winter Olympics, they said Thursday.
The deaths in southern Russia's Stavropol territory, which borders the province where the Olympics will be held in Sochi, have prompted security forces to conduct an anti-terrorism sweep there, state-run RIA Novosti news agency said. It's one of the latest moves to address security concerns before the games.
The bodies of three men and explosive material were found Thursday in a vehicle in Maryinskaya in Stavropol province, Russia's Investigative Committee said on its website.
A day earlier, a vehicle containing a body exploded in Tambukan, in the same province, as police approached it, and two other bodies were found in other vehicles in Zolskaya Wednesday, the Investigative Committee said.
The latest alert comes amid heightened concern about security following twin bombings on public transit in the southern city of Volgograd at the end of last month.
No group has claimed responsibility for those attacks, but suspicion has fallen on Chechen separatist groups.
Three people also died last month in a car bombing in the city of Pyatigorsk, in Stavropol territory.
Dmitri Trenin, director of the Moscow Carnegie Center, said it appeared that the latest killings may be the work of Islamist militants from the tiny North Caucasus republic of Kabardino-Balkaria, which neighbors Stavropol territory.
The militants have carried out similar attacks on taxi drivers and other ordinary people in the past three to four years, he said. They are motivated by a vendetta against the local police and authorities and, more broadly, against the secular Russian state.
The latest security incidents are not likely to be directly linked to the Winter Olympics in Sochi but are part of a bigger picture of unrest in the North Caucasus region, Trenin said. They are receiving more attention than usual because of the upcoming games, he added.
There has been no official confirmation that militants from Kabardino-Balkaria were involved in the suspicious deaths in Stavropol territory.
Cossacks boost Sochi security
A special exclusion zone went into force in Sochi on Tuesday, under which access to the Black Sea resort town is heavily restricted.
About 400 Cossacks arrived Thursday in Sochi, where they will be deployed to assist the police during the games and provide security for visiting athletes and tourists, RIA Novosti said.
They will accompany police patrols in full traditional uniform, including fur hats and swords, the news agency said, in a move that carries echoes of Tsarist Russia. The Cossacks are known for their military prowess.
Security analysts have warned that terrorists targeting the games may try to strike elsewhere in Russia against softer targets than those in Sochi.
Chechen warlord Doku Umarov, leader of the Islamist Caucasus Emirate group, last summer called on his followers to do what they could to disrupt the games.
CNN's Laura Smith-Spark wrote and reported in London and Nic Robertson reported from Moscow. CNN's Alla Eshchenko contributed to this report.