Oleg Alborov, the secretary of the breakaway province's security council, died when a bomb detonated as he opened his garage at 7 a.m. (3 a.m. GMT) Sunday.
Irina Gagloyeva said South Ossetia had sealed off approaches to the Georgian-Ossetian border and was tightening an entry regime after the incident.
Boris Chochiyev, the first vice-premier of the self-proclaimed republic, blamed Georgia for the explosion in Tskhinvali, which killed Alborov.
"We have long stated that the Georgian side is trying to aggravate the situation in the conflict area on the eve of the G-8 summit. The killing of Oleg Alborov, the head of the republic's security council, is a part of this plan," Chochiyev said.
But Georgia has dismissed the accusations.
"The accusations against Georgia are absurd. The Georgian side has nothing to do with this killing," said Shota Khizanishvili, the administration head of the Georgian interior minister.
Georgia's president has sought to return the region to the fold of the country after South Ossetia proclaimed independence in 1990 and fighting broke out. Russia has had peacekeepers stationed in the conflict zone to maintain a ceasefire since 1992.
