The Ethiopian government today ordered the United States to cut the staff of its embassy here by half and close its defense attache office within seven days.

The Ethiopian New Agency, which reported the government action, did not give any reason for the order, but it was the second such action in five weeks by the Marxist government as once-close ties with the United States are being replaced by more Ethiopian reliance on the Soviet Union and Cuba. On April 23, the Ethiopians ordered the closing of all U.S. facilities here except the embassy and the U.S. Agency for International Development (AID) office.

This time the Ethiopian order again made an exception of AID staff, which administers a program of $18.6 million in economic and food assistance, saying they need not be included in the 50 per cent cut.

In Washington, a State Department spokeswoman said the 22-person AID staff in Ethiopia would be reduced anyway, but it has not been determined by how much.

The State Department also said it had informed the Ethiopians of its intention to reduce its Addis Ababa embassy staff before the latest order was issued. In a statement, the department said the Ethiopians had been told the United States would "reduce our staff . . . to bring it down to a level compatible with the reduced scale of our activities following the April expulsion by the Ethiopian government."

[The State Department said that the order means that seven defense attaches and about 30 embassy personnel will have to leave Addis Ababa by June 4. With dependents, the department said, the number of embassy connected personnel leaving will reach about 100. Included in the departing staff will be two-thirds of the embassy's Marine guards, a reduction the Ethiopian order requires.]

The Ethiopian announcement also ordered the closing of British and Egyptian defense attache offices.

The order came two days after the U.S. State Department confirmed a Washington Post report that about 50 Cuban military advisers had arrived in Ethiopia to help the besieged military government fight rebel groups in two sections of the country. The Post report said that as many as 500 more Cuban military experts would be sent here in the next few weeks.

The Cubans are expected to help command and coordinate a big volunteer army the government is forming to fight in northern Eritrea Province and in the eastern Ogaden region, where separatist groups have taken over most of the territory, and against the anti-Marxist Ethiopian Democratic Union that has seized control of most of Begemdir Province in norther Ethiopia.

The Cuban action, along with [WORD ILLEGIBLE] large shipments of Soviet arms to Ethiopia, appears to be part of a Soviet-Cuban plan to compensate the regime of Col. Mengistu Haile [WORD ILLEGIBLE] for the recent loss of military from the United States. Washington decided to shut off her aid because alleged human rights violations.

The new Ethiopian announcement is expected to put in greater jeopardy Mengistu's request for more than [WORD ILLEGIBLE] million worth of ammunition and military equipment from the United States, including some tanks and jets that have already been purchased but not delivered. Support for aid to Ethiopia has been dwindling in the [WORD ILLEGIBLE] Congress and in the Carter administration.

For almost 25 years the United States was Ethiopia's main foreign backer and arms-supplier, but with the deposing of the late Emperor Haile Selassie in 1971 and the [WORD ILLEGIBLE] of the Marxist government, relations between the two countries have progressively worsened.

[United Press International reported that U.S. Embassy officials in Addis Ababa have confirmed that ]WORD ILLEGIBLE[ Fox, 27, field director for Foster Parents Plan International, Inc. is being held there for allegedly carrying anti-government leaflets.]

[Fox, a 1971 graduate of American University in Washington, was arrested Tuesday in a village near the Kenya border during what the government called a sweep for "counter revolutionaries." In Warwick, R.I., a spokesman for Fox's organization called the charge "ludicrous."]