The incident left at least 14 Palestinians dead and dozens wounded, including a number of children.
A spokesman for Mr. Annan issued a statement voicing the Secretary-General's particular concern over reports that an Israeli helicopter gunship fired a missile into a crowd of civilians in "reckless disregard" of the obligation under international humanitarian law to protect the civilian population.
"Such actions have no legal or moral justification," spokesman Fred Eckhard told the press in New York, reiterating Mr. Annan's appeal to both sides to halt all violent and provocative acts, as called for by the Security Council.
"Actions like those carried out this morning do not promote Israeli security," the statement stressed, warning that "they could lead to a further escalation while increasing the sense of vulnerability and insecurity among both the Palestinians and Israelis."
In a related development, the UN Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) today reported that a local hospital was fired on during the attack in Khan Yunis, where students didn't show up for school today because of the violence.