CAIRO, May 28 (Reuters) - Average yields on Egypt's three- and nine-month treasury bills rose at auction on Sunday, data from the central bank showed.

The average yield on the 273-day bill rose to 20.478 percent from 19.709 percent at the previous auction.

The yield on the 91-day bill increased to 20.519 percent from 19.493 percent previously.

(Reporting by Amina Ismail; Editing by Ahmed Aboulenein) ((amina.ismail@thomsonreuters.com; +20 2 2394 8114;))