MOSCOW - Russia's foreign exchange reserves grew last week by 15.4 billion dollars, the central bank said Thursday, in the first rise for several weeks amid a fall in the value of the ruble. The central bank has been selling foreign currency to buy rubles to prop up the national currency, which has been under pressure due to the global financial crisis and fears for the Russian economy. The forex reserves rose to 450.8 billion dollars (322 billion euros) as of December 19 compared with 435.4 billion dollars as of December 12, the central bank said in a statement. The bank had spent tens of billions of dollars over recent weeks supporting the ruble to prevent it suffering a major devaluation in the face of the financial crisis. But it has recently allowed 10 minor devaluations of the ruble by widening the fixed corridor in which it is allowed to move against a currency basket made up of euros and dollars. This has taken the pressure off the central bank to maintain a strong ruble value by buying up rubles. Analysts have said they expect further weakenings worth a total of 10 percent in the weeks to come. President Dmitry Medvedev said Wednesday that the Russian ruble should become "more flexible". "As for the exchange rate of the ruble, it must be effective and it must correspond to the real situation of our economy," he said in a year-end television interview.