Israeli Prime Minister Ehud Olmert expressed regret and "deep condolences" Wednesday over the death of four UN observers in an Israeli air strike on their post in southern Lebanon.

In a phone call to UN Secretary General Kofi Annan, Olmert said that he instructed the Israel Defense Forces to conduct a thorough investigation into the deadly attack which he regarded as amistake, and that he would share the results with him, accordingto a statement issued by Olmert's office.

He also expressed dismay over Annan's earlier comments that the Israeli air raid was intentional.

Annan said that he was "shocked" at Israel's "apparently deliberate targeting" of the UN post.

Annan called on Israeli government to conduct a thorough inquiry into this incident and inform the U.N. of its finding as soon as possible.

Four U.N. military observers were killed in an Israeli airstrike on their post in southern Lebanese border town of Khiam Tuesday night. The victims were from Finland, Austria, Canada and China. Five other U.N soldiers and one military observer were wounded.