South African mobile network operator MTN Group is facing an investigation by a U.S. Department of Justice grand jury into its conduct and that of its former subsidiary in Afghanistan and an Iranian cell phone carrier it partly owns, the company said.

The news and a slight downward revision of its medium-term service revenue outlook for its domestic market sent its shares tumbling 9% on Monday, overshadowing results showing it made a half-year profit following a loss a year earlier.

Group CEO Ralph Mupita told reporters that Africa's biggest telecoms operator had been informed by its external U.S. counsel of the DOJ grand jury investigation, adding that it was cooperating and voluntarily responding to requests for information.

Mupita declined further comment on the case

MTN faces other ongoing legal cases in the United States filed on behalf of American service members and civilians who were injured or killed in Iraq and Afghanistan between 2005 and 2010, who accuse it of violating the U.S. Anti-Terrorism Act.

"The plaintiffs' complaints allege that MTN supported anti-American militias in Iraq and Afghanistan through its participation in Irancell," the company said in a statement,.

MTN owns 49% of Irancell.

"Jurisdictional discovery in the cases, ordered in July 2023, has now closed and the plaintiffs filed an amended complaint on 6 August 2025," the company said.

"The amended complaint now includes additional claims against MTN, which are similar to those asserted in the three other pending ATA (U.S. Anti-Terrorism Act) cases in which MTN is involved. MTN will file a Motion to Dismiss the amended complaint."

MTN shares fell as much as 9% before closing down 8.4%.

"(The) shares have done very well coming into these (half-year) results so part of the selloff could also be profit taking by short-term investors given the U.S. investigations could be an overhang," said Peter Takaendesa, Chief Investment Officer at Mergence Investment Managers.

MTN is also facing challenges in growing service revenue in its home market due to pricing pressures, intense competition and weak consumer spending.

Based on assumptions regarding market conditions and outlook in South Africa, it revised its medium-term guidance for service revenue growth in South Africa to low to mid-single digits from mid-single digits.

It reported headline earnings per share of 645 cents in the six months to June 30, compared to a headline loss of 256 cents a year earlier, as macroeconomic conditions, inflation and foreign exchange rates in Nigeria and Ghana showed improved stability.

Group service revenue grew by 23.2% to 105.1 billion rand ($5.97 billion) as data and fintech revenues increased by 36.5% and 37.3%, respectively.