CAIRO, Dec 3 (Reuters) - The second highest-ranking official in Egypt's central bank has resigned, two senior sources said on Wednesday.

The sources did not say when Nidal Assar, deputy governor for investment and foreign relations, would leave the post. It was not clear who would fill the key role which includes management of Egypt's foreign currency reserves.

The reserves, which reached $16.909 billion in October, are critical to enabling the country of 87 million to import supplies of wheat and gas.

The reserves started falling sharply after the 2011 uprising that ousted President Hosni Mubarak. But they began climbing back again last year when Gulf Arab states sent billions of dollars in aid after the army ousted Mubarak's successor, elected President Mohamed Mursi. (Reporting by Abdelmoneim Drar; Writing by Stephen Kalin; editing by Ralph Boulton)