
The government said in a statement Monday that the days of mourning will be observed starting Tuesday and that during this time the national flag will fly at half-mast.
The statement said that Ethiopia's lawmakers will meet Tuesday to discuss the killings and consider a response.
A 29-minute video believed to be released by the Islamic militants shows at least 35 Ethiopian Christians held captive in Libya being shot or beheaded by extremists.
Redwan Hussein, an Ethiopian government spokesman, said on Sunday that he believed the victims were Ethiopian migrants trying to reach Europe.
Addis Ababa has condemned the killings, and said its embassy in Egypt was trying to confirm exactly how many died and their identities.
"Many of them were Ethiopians, even though we don't know the exact number yet," Communications Minister Redwan Hussein told AFP said, adding that it remained unclear if it would be possible to recover the bodies.
"The Ethiopian government is deeply saddened by the barbarous act committed against our innocent nationals," the Office of Government Communications Affairs said in a statement. Officials were working to identify the victims, it added.
AFP contributed to this report.