Oct 25 (Reuters) - Zain, Kuwait's No.1 telecom operator by subscribers, reported an 18 percent fall in third-quarter profit on Sunday, extending a sustained earnings slump.

The former monopoly, which operates in eight countries in the Middle East and Africa, made a net profit of 38 million dinars ($125.50 million) in the three months to Sept. 30, down from 46 million dinars in the year-earlier period, it said in a statement.

Two analysts polled by Reuters had forecast Zain would make a quarterly profit between 37.0 million dinars and 46.1 million dinars.

The firm had posted falling profits in 10 of the preceding 12 quarters as tougher domestic competition, service interruptions and higher costs in war-torn Iraq and foreign exchange volatility weighed on the bottom line.

Third-quarter revenue was 292 million dinars. This compares with 294 million dinars a year ago.

In Kuwait, Zain competes with Ooredoo Kuwait, a unit of Qatar's Ooredoo, and Viva, an affiliate of Saudi Telecom Co (STC).

($1 = 0.3028 Kuwaiti dinars)

(Reporting by Matt Smith; Editing by Tom Arnold) ((matt.smith1@thomsonreuters.com; 00971506354039; Reuters Messaging: matt.smith1.thomsonreuters.com@reuters.net))