(Adds Ethiopia reaction, paragraphs 6-8)

OSLO, Aug 31 (Reuters) - Norway said it might cut about a third of its direct state aid to Ethiopia after six Norwegian diplomats were expelled from Addis Ababa.

Ethiopia told the diplomats to leave by mid-September and accused Norway of undermining its national security and spreading instability in the Horn of Africa.

Norway's Foreign Minister Jonas Gahr Stoere told daily Aftenposten the annual aid directly administered by Norway's embassy staff in the impoverished African state is about 100 million Norwegian crowns ($17.19 million).

Stoere said this may be reduced by about 30 million crowns, not as "punishment" for the expulsions but "as a consequence of reduced manpower at the embassy", he was quoted as saying.

Most of Norway's aid to Ethiopia, which totalled 268 million crowns last year, was distributed through the United Nations and other organisations, and will not be affected, Stoere said.

Ethiopia had accused Norway of trying to spread instability across the Horn of Africa and undermining its national security. On Friday it said it would never accept such "interference".

"Ethiopia will not and cannot, under any circumstance, abandon its responsibility to maintain its national security," foreign ministry spokesman Wahide Belay said.

"While Ethiopia welcomes development assistance, it should not be at the expense of its national security."

Norwegian media have said Oslo's efforts to help cool tensions between Ethiopia and Eritrea -- which fought a 1998-2000 border war that killed 70,000 people -- appeared to be the cause of Ethiopia's expulsion of the diplomats.

A Norwegian official said Ethiopian officials were upset by Norwegian attempts to arrange a meeting of the Ethiopia-Eritrea border commission in September. (Additional reporting by Tsegaye Tadesse in Addis Ababa)

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