JOHANNESBURG (Reuters) -MTN Group, Africa's biggest telecoms operator, reported an 18.8% fall in its first-quarter service revenue, weighed down by the performance of MTN Nigeria.

South Africa-headquartered MTN, with 288 million subscribers in 18 markets across Africa, said its reported group service revenue fell to 42.9 billion rand ($2.34 billion) in the quarter ended March 31, from 52.8 billion rand in the same quarter last year.

In constant currency, service revenue, which excludes device and SIM card revenue, rose by 11.1%.

Its service revenue from South Africa surpassed that of Nigeria, its biggest market by revenue, growing marginally by 3% to 10.4 billion rand, while Nigeria tumbled by 52.8% to 10.2 billion rand.

"The macro environment in the first quarter of 2024 remained challenging with ongoing high inflation as well as local currency devaluations in some of our key markets," Group President and CEO Ralph Mupita said in a statement.

Overall reported group earnings before interest, tax, depreciation and amortization (EBITDA) fell by 28.7% to 17.2 billion rand and rose by 3.9% in constant currency.

Reported EBITDA margin declined by 5.8 percentage points to 37.9% due to rising costs and currency depreciation mainly in Nigeria.

($1 = 18.3539 rand)

(Reporting by Nqobile Dludla; Editing by Kim Coghill and Subhranshu Sahu)