An article Monday in The Washington Post incorrectly stated that Somali Ambassador Abdullahi Ahmed Addou is the son-in-law of Somali President Siad Barre. The two are not related.

Somalia is charging that the United States badly misled it about supplying arms aid in what has turned into Africa's latest war, in Ethiopia's region, Newsweek reports in its current issue.

The Somalian ambassador to the United Nations, Abdirizak Haji Hussein, last week berated Richard M. Moose Jr., assistant secretary of State for African affairs, about American "unreliability," the magazine reported.

At a reception given by American U.N. Ambassador Andrew Young, Hussein reportedly told Moose:

"You let us down badly by misleading us. You have thrown away a unique opportunity to . . . establish your influence in the Horn of Africa."

The report said Somalia privately claims it began an all-out offensive against Ethiopia's disputed Ogaden area in July because of the prospect of American aid, and a secret U.S. message which it interpreted "as a go-ahead to conquer the area," ostensibly by guerrilla warfare. Carter administration officials deny expressing any such intention.

President Carter, early in his administration, cited Somalia as one of "crucial" regions where he wanted to challenge the Soviet Union for influence. When Ethiopia expelled U.S. military advisers in April and put its reliance on the Soviet Union, the United States joined with France and Britain to explore arms aid for Somalia, also a Soviet weapons client.

According to Newsweek, Somalia President Muhammad Siad Barre sent his American physician and adviser, Dr. Kevin Cahill, to confer with State Department counselor Matthew Nimetz. The report said Cahill reported back to Siad Barre in June that the United States were interested in arms aid and was "not averse to further guerrilla pressure in the Ogaden."

In return, the report said, the United States wanted Somalia to drop its territorial claims to parts of northern Kenya and to the former French colony of Djibouti.

The account said Siad Barre's son-in-law, the Somali ambassador to Washington Abdullahi Ahmad Addou, conferred twice with President Carter and reported that the Cahill message was legitimate.

But as the fighting in the Ogaden reached major proportions, State Department spokesman Hodding Carter III last month said the Carter administration had decided that "providing arms at this time would add fuel to a fire we are most interested in putting out."

Newsweek's Arnaud de Borchgrave quoted Moose as saying that "our assurances" of interest in meeting Somalia's "legitimate defensive requirements" for arms in earlier months "were not of such a nature that a prudent man would have mounted an offensive on the basis of them."

Nimetz was quoted as denying he gave Cahill a message of encouragement for the Ogaden offensive. However, the account added, Nimetz said, "It is quite possible that Siad Barre got the impression that we didn't care about the Ogaden."

The Newsweek account said an unidentified senior State Department official described the role of American diplomacy with Somalis as "a classic case of incompetence and mismanagement."