Russian President Dmitry Medvedev said on Friday Russia would take action to protect its citizens under siege by Georgian troops driving to re-assert control over the breakaway region of South Ossetia.
Medvedev did not specify what action Moscow might take.
"We will not let the deaths of our citizens go unpunished, the guilty will incur the punishment they deserve," Medvedev was quoted by news agency Interfax as saying in his first statement since heavy fighting erupted on Thursday in the region claiming casualties on both sides.
"Under the Constitution and the federal law as president of the Russian Federation, I am committed to protecting a life and dignity of Russian citizens wherever they may be," Medvedev said.
Most residents in South Ossetia and Georgia's other breakaway region of Abkhazia have been issued Russian passports, and Moscow has moved to strengthen ties with the rebel governments in recent years.
The news agency Interfax cited eyewitnesses as saying they saw Russian troops moving into the separatist capital of Tskhinvali.
The military convoys are being sent to reinforce peacekeeping forces in the region; unnamed local residents were quoted as saying.
In a televised speech earlier on Friday, Georgian President Mikheil Saakashvili accused Russia of arming separatists and bombing Georgian villages. Saakashvili ordered a full military mobilization.