CAIRO, June 18 (Reuters) - Average yields on Egypt's three-month treasury bills dipped while yields on nine-month bills rose at an auction on Sunday, data from the central bank showed.

The average yield on the 266-day bill rose slightly to 20.350 percent from 20.317 percent at the last sale.

The yield on the 91-day bill dipped to 20.244 percent from 20.263 percent at the previous auction.

(Reporting by Arwa Gaballa; Editing by Mark Potter) ((arwa.gaballa@thomsonreuters.com; +20 2 2578 3290;))