Mr. Annan "is concerned that such an action would lead to further bloodshed and death and acts of revenge and retaliation," a UN spokesman said in a statement.
"He reiterates that extrajudicial killings are against international law and calls on the Government of Israel to immediately end this practice," spokesman Fred Eckhard added. "The only way to halt an escalation in the violence is for the parties to work towards a viable negotiating process aimed at a just, lasting and comprehensive settlement."
Speaking directly to the press earlier this morning, Mr. Annan said, "I do condemn the targeted assassination of Sheikh Yassin and the others who died with him. Such actions are not only contrary to international law, but they do not do anything to help the search for a peaceful solution."
He also appealed to "all in the region to remain calm and avoid any further escalation in tensions."
With the diplomatic Quartet of the UN, European Union, Russian Federation and United States meeting today in Cairo, the Secretary-General said the assassination "has complicated issues." He added that he spoke this morning to his envoy, Terje Roed-Larsen, who is in Egypt with the other representatives, and they were assessing the impact of the latest actions.
"As I have indicated earlier, it doesn't really facilitate the task of peacemakers," Mr. Annan said.
Mr. Roed-Larsen himself also issued a statement strongly condemning Israel's action. "In addition to the killing of so many, the assassination of Ahmad Yassin threatens the tenuous steps currently underway to revive the peace process," he said.
"Only a viable peace process can bring about a halt to violence," stressed Mr. Roed-Larsen, the UN's Special Coordinator for the Middle East Peace Process.
Also reacting to the latest developments, the Acting UN High Commissioner for Human Rights, Bertrand Ramcharan, expressed deep concern over Israel's continued use of assassination in the occupied Palestinian territory. In a statement issued in Geneva, Mr. Ramcharan said that while there is no doubt that Israel has a right to defend itself, this must be done within the rule of law.