UN Secretary-General Kofi Annan said on Sunday that questions still surrounding the deaths of the four UN military observers in Lebanon must be answered.

"The men we lost last Tuesday came from different nations, but worked together to carry out the difficult mission entrusted to them by the United Nations," he said in a message to a memorial service held at the Headquarters of the UN Truce Supervision Organization in Jerusalem," he said.

"The United Nations will be forever in their debt," he added.

The ceremony was held to honor the memory of Du Zhaoyu, Paeta Hess-Von Kruedener, Hans-Peter Lang and Jarno Makinen, who lost their lives during an Israeli bombardment last week at Patrol BaseKhiyam in Southern Lebanon.

"Many questions surround the tragic deaths of these unarmed peacekeepers, and these must be answered," Annan said in the message.

"That will not bring them back to their loved ones, but it will help us to better ensure the safety of other peacekeepers, now and in the future. Meanwhile, we must honor their work by ensuring that the death and destruction come to an end as soon as possible, and by bringing help to all those whose lives are being torn apart," he said.