KARACHI (Reuters) - Pakistan's foreign exchange reserves fell to $16.39 billion in the week ending Dec. 18, down from $16.41 billion the previous week, the State Bank said on Thursday. Reserves held by the State Bank of Pakistan (SBP) fell to $12.63 billion from $12.69 billion in the week ending Dec. 18, while those held by commercial banks rose to $3.76 billion from $3.72 billion, said Syed Wasimuddin, chief spokesman of the central bank. Pakistan's reserves hit a record high of $17.10 billion in the week ending Oct. 15 because of an increase in remittances from overseas Pakistanis and a narrowing trade deficit. Earlier the reserves got a boost in September after the IMF sent Pakistan $450 million and said that the money would go toward the budget to help with additional spending for flood relief and immediate foreign exchange needs. This was separate from the $11 billion IMF bailout programme, agreed in 2008. In May, Pakistan received $1.13 billion in the fifth tranche of the programme. Pakistan has formally asked for a nine-month extension of its $11 billion IMF bailout package, a source involved with the IMF talks said on Monday, but the extension has not been approved yet.