MOSCOW, August 8 (RIA Novosti) - President Dmitry Medvedev said on Friday that Russia will punish those guilty of killing Russian nationals in South Ossetia, where Georgia has been waging an offensive to regain control of the breakaway region.

Speaking at an emergency Russian Security Council session focused on the violence, Medvedev said: "As president, I am obliged to protect the lives of our citizens wherever they may be. We will not let those guilty of killing our nationals evade punishment."

Medvedev said civilians are being killed in South Ossetia, most of whom have Russian citizenship. "Georgian troops in fact staged an act of aggression against Russian peacekeepers and civilians in South Ossetia last night."

Georgia launched a large-scale offensive against the breakaway province during the night, using tanks, combat aircraft, heavy artillery and infantry.

Separatist officials said the regional capital, Tskhinvali, has suffered major destruction, and reported 15 civilian deaths. Earlier reports also said several Russian peacekeepers were killed and wounded in a Georgian attack. Georgia said three troops were killed and about 17 wounded by separatist forces.

The Georgian offensive came the morning after Georgian President Mikhail Saakashvili declared a unilateral ceasefire. Georgian Prime Minister Lado Gurgenidze said Georgia was forced to attack after separatists shelled Georgian villages.

The Russian president said Georgian peacekeepers had targeted Russian peacekeepers during Friday's attacks. "Georgian peacekeepers fired at Russian troops, with whom they were supposed to ensure peace in the region."

The peacekeeping force in South Ossetia comprises troops from Georgia, Russia, and the breakaway republic.

President Saakashvili, who seeks NATO membership for Georgia, has pledged to bring South Ossetia, home to around 80,000 people, along with breakaway Abkhazia, back under central control, and has accused Russia of trying to annex the regions.

Medvedev said Russia would remain a guarantor of security in the Caucasus, in a reference to Tbilisi's demands that Russian troops pull out from the conflict zones.

"Historically Russia was and will remain the guarantor of security for the peoples of the Caucasus," he said.

Prime Minister Vladimir Putin, speaking in Beijing earlier on Friday, said it would be difficult to stop Russian volunteers from heading to South Ossetia to help the province.

Volunteers to fight on South Ossetia's side have been gathering in neighboring Russian Caucasus republics. Abkhazia has also pledged its support.