CAIRO, July 20 (Reuters) - Average yields on Egypt's three-month and nine-month treasury bills fell at an auction on Thursday, data from the central bank showed, amid high foreign buying of Egyptian debt.

The average yield on the 91-day bill slipped to 21.564 percent from 21.920 percent at the last similar auction on July 16 and the yield on the 273-day bill fell to 21.582 percent from 21.775 percent.

Foreign buying accounted for 32 percent of Thursday's sale, standing at 4.5 billion Egyptian pounds ($252.10 million), said Sami Khallaf, the head of public debt at the finance ministry.

Demand for Egypt's domestic debt has increased since the central bank raised key interest rates by 2 percentage points earlier this month, its third increase since Egypt floated its currency in November last year.

($1 = 17.8500 Egyptian pounds)

(Reporting by Eric Knecht; writing by Arwa Gaballa) ((arwa.gaballa@thomsonreuters.com; +20 2 2578 3290;))