MOGADISHU, Sept 9 (Reuters) – Somalia’s two most powerful leaders were locked in a standoff on Wednesday after they named different men to head the intelligence service of the politically unstable nation in the Horn of Africa.

The row between President Mohamed Abdullahi Mohamed and Prime Minister Mohammed Hussein Roble, nominally over a murder investigation, marks an escalation of months of tension between them in a country riven by militant attacks and clan rivalries.

On Monday, Roble suspended Fahad Yasin, director of the National Intelligence Service Agency, saying he had failed to deliver a report on the case of an agent who disappeared in June.

Roble appointed another man, Bashir Mohamed Jama, as interim head of the agency.

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But the president called Roble’s move unconstitutional and named a third man, Yasin Abdullahi Mohamed, as head on Tuesday.

The president’s appointee took over at a handover ceremony on Wednesday, the agency said on Twitter. Security around its headquarters was tight, local residents said.

Later on Wednesday Roble appointed former finance minister Abullahi Mohamed Nur as the new internal security minister, his office said in a statement. Nur is currently a lawmaker who has been critical of the president.

Subsequently on Wednesday the president said in a statement that the dismissal of the minister and the appointment of a new one was unconstitutional, making the change “null and void”.

On Tuesday, the African Union, the United Nations and foreign donor nations, including Britain and the United States, had urged a de-escalation of the row, asking the two leaders to “avoid any actions that could lead to violence”.

Roble and Mohamed had clashed in April, when the president unilaterally extended his four-year term by two years, prompting army factions loyal to each man to seize rival positions in the capital, Mogadishu.

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The confrontation was resolved when the president put Roble in charge of security and organising delayed legislative and presidential elections. That process was supposed to be concluded next month but several days ago was pushed back again.

In a statement on Tuesday, the president named Yasin, the man Roble had sacked, as his personal security adviser.

Also on Tuesday, Roble accused Mohamed of “obstructing effective investigation” of the case of Ikran Tahlil Farah, the agent who went missing while working in the cyber security department of the intelligence agency.

Her family have said publicly they believe she was murdered, and they hold the agency responsible.

The agency has not responded to the family’s accusation.

On Wednesday the family filed a case in a military court seeking an arrest warrant for four NISA officials, including the sacked boss Yasin, whom they hold responsible for Farah’s disappearance, lawmaker Mahad Mohamed Salad told Reuters.

(Reporting by Abdi Sheikh; Writing by Duncan Miriri; Editing by Mark Porter and Clarence Fernandez)