MOSCOW (Reuters) - Russia's gold and foreign currency reserves dropped below $400 billion for the first time since August 2009 as of Dec. 19, the central bank said on Thursday.

The central bank has spent over $80 billion defending the rouble this year, as a sharp slide in oil prices and Western sanctions over the Ukraine crisis have driven the Russian currency sharply lower, threatening financial stability.

The bank has expanded its foreign-currency repurchase agreement operations in response to market instability, offering banks foreign currency for a period of one week, 28 days and one year, to ease a deficit of dollars and euros.

Analysts said around $5 billion of the $15 billion fall in the week to Dec. 19 was because of interventions to support the rouble, while around $7 billion was due to foreign currency loaned to banks as part of repo operations.

Since that foreign currency will be returned to the central bank at a later stage, the reserves could recover some of the losses in the near or mid-term, they said, adding that the remainder of the losses were likely due to shifts in the value of the bank's foreign-currency holdings.

The central bank provided the following figures for its gold and foreign exchange reserves on Thursday (in billion dollars):

Latest week 398.9

Previous week 414.6

End-2013 509.6

NOTE - The reserves include monetary gold, special drawing rights, reserve position at the IMF and foreign exchange.

(Reporting by Alexander Winning, Elena Fabrichnaya and Vladimir Abramov)