(Adds details, background) By Tsegaye Tadesse ADDIS ABABA, Aug 28 (Reuters) - Ethiopia accused Norway on Tuesday of undermining its national security and spreading instability in the Horn of Africa, a day after the Addis Ababa government asked Oslo to recall six diplomats. "Norway has consistently and repeatedly (been) involved in acquiring leverage with the focus of causing instability in the Horn of Africa region," said Wahide Belay, spokesman for Ethiopia's Foreign Ministry. "This, in turn, has seriously aggravated problems of stability in the Horn of Africa region and also threatened to undermine Ethiopia's national security. ... It has now become clear that there has been no change of policy in this regard." Norwegian media said Oslo's efforts to help reduce tensions between Ethiopia and Eritrea -- which fought a 1998-2000 border war that killed 70,000 people -- appeared to be to blame. A Norwegian official said Ethiopian officials were upset by Norwegian attempts to arrange a meeting of the Ethiopia-Eritrea border commission in September. "They reacted very badly to that," Raymond Johansen, state secretary in Norway's Foreign Ministry, was quoted telling Norwegian news agency NTB. Tensions still run high between Asmara and Ababa over the disputed frontier. Late on Monday, Norway's foreign minister, Jonas Gahr Stoere, said in a statement that his government regretted the "unilateral decision" taken by the authorities in Addis Ababa. "We must now take the consequences of the fact that the Ethiopian authorities are upholding their decision and we are therefore making preparations to withdraw six diplomats from our embassy in Addis Ababa (by Sept. 15)," Stoere said. Stoere did not say whether Ethiopia provided any background on the reasons for the expulsions. Stoere said the "sharp reduction in staffing" meant Norway might not be able to maintain development cooperation with Ethiopia at current levels, he said. Oil-rich Norway is one of the world's biggest aid donors in per capita terms. Wahide said Ethiopia's government remained ready to talk. "Our efforts have been and still are to engage Norway's government in serious dialogue over the matter," Wahide said. (Additional reporting by Wojciech Moskwa in Oslo)