Each side said the other was looking to take advantage of heightened political tensions in Georgia, where the opposition has been protesting in the streets for a week demanding the resignation of President Mikheil Saakashvili.
Russia sent troops into South Ossetia to crush a Georgian assault on the separatist region in August last year, and then officially recognised it and another rebel region of Georgia - Abkhazia - as independent states.
A European Union mission monitoring the ceasefire said it had registered Russian reinforcements at the boundaries between Georgia-controlled territory and South Ossetia and Abkhazia, but had not monitored any significant Georgian build-up.
EU spokesman Stephen Bird said the "significant" Russian reinforcements at the boundary of Abkhazia appeared to have moved back on Thursday.
Asked if they had seen a similar pullback in South Ossetia, Bird said: "Not as far as we have noticed yet."
A confidential assessment compiled by EU diplomats in Georgia and seen by Reuters said the Russian reinforcements included tanks, armoured personnel carriers, artillery and "Grad" multiple-rocket launchers.
Volatile border
"Thus the situation at the ABL (administrative boundary line) remains in flux and volatile as Russian/South Ossetian forces continue to establish new facts on the ground," said the the assessment, dated 13 April.
Georgian Interior Ministry spokesman Shota Utiashvili said the Russian military had brought in extra forces, "but they never withdrew the old forces".
"We are concerned about this," he said. Utiashvili said Georgia believed an extra 5,000 Russian forces had entered both regions before the protests.
Russia said it had taken "precautionary measures" to ensure security in South Ossetia and Abkhazia during the tense period, adding it had concrete information about likely provocations.
Foreign ministry spokesman Andrei Nesterenko said Russian forces were conducting exercises in order to dissuade Tbilisi from any military action, which he suggested would be used to distract attention from opposition protests.
"We have repeatedly come up against the practice, which has now become customary for official Tbilisi, to search for a way out of internal problems by using external aggression," he said.
"The real danger for the stability of the region is the continued remilitarisation of Georgia including the concentration of special forces and military equipment close to the territories of Abkhazia and South Ossetia."
The Georgian Interior Ministry said it had arrested an activist of the Russian pro-Kremlin youth movement 'Nashi' who had crossed into Georgian territory from South Ossetia, and accused him of trying to provoke a confrontation.
The ministry said other 'Nashi' activists were still in the South Ossetian capital Tskhinvali, having travelled from Moscow.
(EurActiv with Reuters.)
