By Tsegaye Tadesse

ADDIS ABABA, Oct 19 (Reuters) - A U.N. official arrested in Mogadishu by intelligence officers this week is under investigation for unspecified crimes, Somalia's prime minister said on Friday.

In the government's first comments on an arrest which infuriated the United Nations and prompted its World Food Programme (WFP) to halt food distribution in the Horn of Africa country, Prime Minister Ali Mohamed Gedi gave few details.

"I know this event has frustrated the United Nations in Somalia and our partners. He will be released if he is found free of the charges against him," Gedi told reporters in Addis Ababa, where he is visiting the Ethiopian government.

More than 60 Somali national security officers stormed U.N. offices in the capital Mogadishu on Wednesday and arrested WFP country director Idris Osman. The U.N. has called the raid a violation of its immunity and demanded his immediate release.

Gedi declined to give more details of the charges against Osman, saying: "He is still under investigation."

The prime minister, who was due to return to Somalia on Saturday, is in the middle of his latest battle for political survival.

FEUD

President Abdullahi Yusuf and his allies are trying to push Gedi out of office as a long-running feud between the two leaders flares up again. Yusuf and his allies failed last year to win a no-confidence vote against Gedi.

On Friday he vowed that parliament would decide whether the president's move to oust him is legal, attempting to quash speculation he would quit.

"There is a difference between myself and the president due to different interpretation regarding an article in the charter of the transitional government," Gedi said.

"The problem of the interpretation will be decided in parliament soon, but our difference cannot sterilise the activities of the transitional government."

Yusuf argues Gedi's term of office was up a week ago, and that he has failed to do the tasks the charter requires. But Gedi and his allies argue he has another 14 months.

The matter ought to be decided by Somalia's chief justice, but the attorney general, an ally of Yusuf, last month ordered him jailed on corruption charges -- so the Somali parliament will decide.

Parliament is due to resume debate on Saturday in Baidoa, the south-central trading town where it sits in a converted warehouse.

The political feud, combined with an Islamist insurgency in Mogadishu, has helped stall the Somali government's progress. The interim government is Somalia's 14th attempt at establishing a national authority since anarchy broke out after the 1991 overthrow of dictator Mohamed Siad Barre.

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