The reserves, the world's third largest, fell from a record $597.5 billion the previous week to $581.1 billion in the week ended Aug. 15, the Central Bank said Thursday.
The decline was the biggest since August 2006, when Russia paid $22.3 billion in debt to the Paris Club, triggering a fall of $16.5 billion in reserves.
"The size of the decline surprised us," said Ronald Smith, chief strategist at Alfa Bank. "We thought it might be half that much."
The news of the reserves falling came after Finance Minister Alexei Kudrin said Sunday that the Georgian conflict had led to capital outflow of $7 billion during Aug. 8-11.
The unexpected strengthening of the dollar against the euro also played a significant role, analysts said.
"While there was a substantial amount of capital flight in the week between Aug. 8 and Aug. 16, much of the volume was due to Russian investors decreasing ruble exposure by converting them into dollars, but not necessarily taking the money out of the country," Smith said.
Vladimir Osakovsky, an economist at UniCredit Aton, estimated that about $5 billion of the decline in reserves was down to the euro's fall against the dollar, while another $1 billion was due to falling gold prices.
About 50 percent of Russia's international reserves are placed in dollar assets while 40 percent are in euros. Another 9 percent, mainly gold reserves, are denominated in pounds sterling.
The ruble fell as much as 0.3 percent against the dollar/euro basket Thursday. It was at 29.6439 by 5:36 p.m., down from 29.5994 Wednesday.
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