MTN boosts Q3 subscribers by 5 pct to 108.4 mln

* Users in Nigeria up 5 pct to 28.7 mln

* Iran subscribers up 8 pct to 20.7 mln

* South Africa base declines

JOHANNESBURG, Oct 29 - Africa's biggest mobile operator MTN <MTNJ.J> boosted subscribers by 5 percent in the third quarter thanks to strong growth in key markets Nigeria and Iran, and expects zero subscriber growth for the year in its local operation.

The company, which operates mobile phone networks across Africa and the Middle East, said it had 108.4 million subscribers at the end of September compared to 103.2 million.

In Nigeria, Africa's most populous nation and MTN's biggest market, subscribers roe 5 percent to 28.7 million.

MTN's Iran unit, launched in late 2006, boosted subscribers by 8 percent to 20.7 million. Its customer base in its second-biggest market South Africa declined to 16.4 million from 17.2 million.

"MTN has revised its subscriber net addition guidance for the year for South Africa to zero," the company said in a statement.

The Ghana operation increased its user base by 2.6 percent, despite competition in the market.