The BBC has a Q&A on this latest incident.
Escalating tensions between Georgia and its breakaway province of South Ossetia have erupted into serious fighting.
The separatist administration in South Ossetia has been trying to gain formal independence since breaking away in a civil war in the 1990s.
Russia has troops in the region, on a peacekeeping mandate. But Moscow also supports the separatists.
What is the status of South Ossetia?
South Ossetia has run its own affairs since fighting for independence from Georgia in 1991-92, in the aftermath of the collapse of the Soviet Union.
It has declared independence, though this has not been recognised by any other country.
Georgian President Mikhail Saakashvili has vowed to bring South Ossetia and another breakaway region, Abkhazia, back under full Georgian control.
Why do Ossetians want to break away?
The Ossetians are a distinct ethnic group originally from the Russian plains just south of the Don river. In the 13th Century, they were pushed southwards by Mongol invasions into the Caucasus mountains, settling along the border with Georgia.
South Ossetians want to join up with their ethnic brethren in North Ossetia, which is an autonomous republic within the Russian Federation.
Ethnic Georgians are a minority in South Ossetia, accounting for less than one-third of the population.
But Georgia rejects even the name, South Ossetia, preferring to call it by the ancient name of Samachablo, or Tskhinvali, after its main city.
What triggered the latest crisis?
Tension has risen since the election of President Saakashvili in 2004. He offered South Ossetia dialogue and autonomy within a single Georgian state - but in 2006 South Ossetians voted in an unofficial referendum to press their demands for complete independence.
In April 2008 Nato said Georgia would be allowed to join the alliance at some point - angering Russia, which opposes the eastward expansion of Nato. Weeks later, Russia stepped up ties with the separatists in Abkhazia and South Ossetia.
In July Russia admitted its fighter jets entered Georgian airspace over South Ossetia to "cool hot heads in Tbilisi". Occasional clashes escalated, until six people were reportedly killed by Georgian shelling. Attempts to reach a ceasefire quickly collapsed.
Could Russia become directly involved in war?
Russia insists it has been acting as a peacekeeper in South Ossetia, rejecting Georgian accusations that it has been supplying arms to the separatists.
However, it has vowed to defend its citizens in South Ossetia - of which there are many. More than half of South Ossetia's 70,000 citizens are said to have taken up Moscow's offer of a Russian passport.
Russia may view limited military intervention as less risky than recognising South Ossetia's independence, which could lead to all-out war with Georgia.
What about Georgia's links to Nato?
President Saakashvili has made membership of Nato one of his main goals. Georgia has a close relationship with the United States and has been cultivating ties with Western Europe.
There are those who believe that Mr Saakashvili may be hoping to draw Nato into a conflict with Moscow, making their alliance a formal one.
But analysts say it is difficult to imagine Nato allowing itself to be drawn into a direct conflict with its Cold War rival after managing to avoid that for so long.