Annan demanded that Israel respect humanitarian law
United Nations Secretary General Kofi Annan has condemned Israel's offensive in the West Bank and Gaza as tantamount to all-out war.
Mr Annan has become increasingly frustrated and distressed at the escalation of violence in Israel.
Last week, for the first time, the secretary general described the Israeli occupation of Arab lands as "illegal" - his strongest public statement to date on the 18-month-old Palestinian uprising.
Now the BBC has obtained a copy of a letter sent by Mr Annan to Mr Sharon.
'Disturbing patterns'
Dated 12 March, the letter speaks of the "disturbing patterns" in the treatment of civilians and humanitarian relief workers by the Israeli Defence Forces.
Mr Annan goes on to say that "judging by the means employed, such as F-16 fighter bombers, helicopter and naval gunships, missiles and bombs, the fighting has come to resemble all-out conventional warfare".
Israel is fully entitled to defend itself against terror, Mr Annan writes, but this right does not discharge it from its obligation to respect fundamental principles and rules on international humanitarian law.
Mr Annan also expressed dismay at Israel's failure to protect and respect ambulances and medical personnel.
Israeli claims that ambulances were being used to smuggle militants and weapons are described by Kofi Annan as "unfounded and unsubstantiated".