In a statement released by his spokesman on Friday, Annan "urged a peaceful resolution of the situation and called for respect for the electoral outcome".
Ethiopian police this week cracked down on protesters disputing the results of the country's 15 May elections. At least 700 people were arrested this week and witnesses reported that police fired into crowds on Wednesday, killing at least 27 and wounding hundreds.
Annan, who recently returned from a diplomatic trip to Ethiopia, has this week been in contact with prime minister Meles Zenawi and opposition leaders, who earlier on Friday signed a non-violence pact over the disputed elections, the statement said.
However, prospects for the agreements appeared dim, as the two sides traded new accusations of malfeasance immediately after it was sealed.
The clashes this week in Addis Ababa mark the worst violence in the Ethiopian capital since student riots in 2001 in which more than 30 people were killed.